how to set div width so left and right padding is equal

Question 1

If you don't mind using a bit of jQuery, the Masonry script does a nice job of this:

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<style>

#container {border:5px solid red;padding:2%;text-align:center;}
 .inner {margin:0 auto;padding:10px;border:5px solid green;display:inline-block;
 text-align:left;}
 .item {width:250px;height:250px;margin:5px;border:1px solid blue;display:inline-block;}

</style>
</head>
<body>

<div id="container">
 <div class="inner">
  <div class="item">ITEM 1</div>
  <div class="item">ITEM 2</div>
  <div class="item">ITEM 3</div>
  <div class="item">ITEM 4</div>
  <div class="item">ITEM 5</div>
  <div class="item">ITEM 6</div>
  <div class="item">ITEM 7</div>
  <div class="item">ITEM 8</div>
 </div>
</div>

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<script>
/**
 * jQuery Masonry v2.1.08
 * A dynamic layout plugin for jQuery
 * The flip-side of CSS Floats
 * http://masonry.desandro.com
 *
 * Licensed under the MIT license.
 * Copyright 2012 David DeSandro
 */
(function(e,t,n){"use strict";var r=t.event,i;r.special.smartresize={setup:function(){t(this).bind("resize",r.special.smartresize.handler)},teardown:function(){t(this).unbind("resize",r.special.smartresize.handler)},handler:function(e,t){var n=this,s=arguments;e.type="smartresize",i&&clearTimeout(i),i=setTimeout(function(){r.dispatch.apply(n,s)},t==="execAsap"?0:100)}},t.fn.smartresize=function(e){return e?this.bind("smartresize",e):this.trigger("smartresize",["execAsap"])},t.Mason=function(e,n){this.element=t(n),this._create(e),this._init()},t.Mason.settings={isResizable:!0,isAnimated:!1,animationOptions:{queue:!1,duration:500},gutterWidth:0,isRTL:!1,isFitWidth:!1,containerStyle:{position:"relative"}},t.Mason.prototype={_filterFindBricks:function(e){var t=this.options.itemSelector;return t?e.filter(t).add(e.find(t)):e},_getBricks:function(e){var t=this._filterFindBricks(e).css({position:"absolute"}).addClass("masonry-brick");return t},_create:function(n){this.options=t.extend(!0,{},t.Mason.settings,n),this.styleQueue=[];var r=this.element[0].style;this.originalStyle={height:r.height||""};var i=this.options.containerStyle;for(var s in i)this.originalStyle[s]=r[s]||"";this.element.css(i),this.horizontalDirection=this.options.isRTL?"right":"left";var o=this.element.css("padding-"+this.horizontalDirection),u=this.element.css("padding-top");this.offset={x:o?parseInt(o,10):0,y:u?parseInt(u,10):0},this.isFluid=this.options.columnWidth&&typeof this.options.columnWidth=="function";var a=this;setTimeout(function(){a.element.addClass("masonry")},0),this.options.isResizable&&t(e).bind("smartresize.masonry",function(){a.resize()}),this.reloadItems()},_init:function(e){this._getColumns(),this._reLayout(e)},option:function(e,n){t.isPlainObject(e)&&(this.options=t.extend(!0,this.options,e))},layout:function(e,t){for(var n=0,r=e.length;n<r;n++)this._placeBrick(e[n]);var i={};i.height=Math.max.apply(Math,this.colYs);if(this.options.isFitWidth){var s=0;n=this.cols;while(--n){if(this.colYs[n]!==0)break;s++}i.width=(this.cols-s)*this.columnWidth-this.options.gutterWidth}this.styleQueue.push({$el:this.element,style:i});var o=this.isLaidOut?this.options.isAnimated?"animate":"css":"css",u=this.options.animationOptions,a;for(n=0,r=this.styleQueue.length;n<r;n++)a=this.styleQueue[n],a.$el[o](a.style,u);this.styleQueue=[],t&&t.call(e),this.isLaidOut=!0},_getColumns:function(){var e=this.options.isFitWidth?this.element.parent():this.element,t=e.width();this.columnWidth=this.isFluid?this.options.columnWidth(t):this.options.columnWidth||this.$bricks.outerWidth(!0)||t,this.columnWidth+=this.options.gutterWidth,this.cols=Math.floor((t+this.options.gutterWidth)/this.columnWidth),this.cols=Math.max(this.cols,1)},_placeBrick:function(e){var n=t(e),r,i,s,o,u;r=Math.ceil(n.outerWidth(!0)/this.columnWidth),r=Math.min(r,this.cols);if(r===1)s=this.colYs;else{i=this.cols+1-r,s=[];for(u=0;u<i;u++)o=this.colYs.slice(u,u+r),s[u]=Math.max.apply(Math,o)}var a=Math.min.apply(Math,s),f=0;for(var l=0,c=s.length;l<c;l++)if(s[l]===a){f=l;break}var h={top:a+this.offset.y};h[this.horizontalDirection]=this.columnWidth*f+this.offset.x,this.styleQueue.push({$el:n,style:h});var p=a+n.outerHeight(!0),d=this.cols+1-c;for(l=0;l<d;l++)this.colYs[f+l]=p},resize:function(){var e=this.cols;this._getColumns(),(this.isFluid||this.cols!==e)&&this._reLayout()},_reLayout:function(e){var t=this.cols;this.colYs=[];while(t--)this.colYs.push(0);this.layout(this.$bricks,e)},reloadItems:function(){this.$bricks=this._getBricks(this.element.children())},reload:function(e){this.reloadItems(),this._init(e)},appended:function(e,t,n){if(t){this._filterFindBricks(e).css({top:this.element.height()});var r=this;setTimeout(function(){r._appended(e,n)},1)}else this._appended(e,n)},_appended:function(e,t){var n=this._getBricks(e);this.$bricks=this.$bricks.add(n),this.layout(n,t)},remove:function(e){this.$bricks=this.$bricks.not(e),e.remove()},destroy:function(){this.$bricks.removeClass("masonry-brick").each(function(){this.style.position="",this.style.top="",this.style.left=""});var n=this.element[0].style;for(var r in this.originalStyle)n[r]=this.originalStyle[r];this.element.unbind(".masonry").removeClass("masonry").removeData("masonry"),t(e).unbind(".masonry")}},t.fn.imagesLoaded=function(e){function u(){e.call(n,r)}function a(e){var n=e.target;n.src!==s&&t.inArray(n,o)===-1&&(o.push(n),--i<=0&&(setTimeout(u),r.unbind(".imagesLoaded",a)))}var n=this,r=n.find("img").add(n.filter("img")),i=r.length,s="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///ywAAAAAAQABAAACAUwAOw==",o=[];return i||u(),r.bind("load.imagesLoaded error.imagesLoaded",a).each(function(){var e=this.src;this.src=s,this.src=e}),n};var s=function(t){e.console&&e.console.error(t)};t.fn.masonry=function(e){if(typeof e=="string"){var n=Array.prototype.slice.call(arguments,1);this.each(function(){var r=t.data(this,"masonry");if(!r){s("cannot call methods on masonry prior to initialization; attempted to call method '"+e+"'");return}if(!t.isFunction(r[e])||e.charAt(0)==="_"){s("no such method '"+e+"' for masonry instance");return}r[e].apply(r,n)})}else this.each(function(){var n=t.data(this,"masonry");n?(n.option(e||{}),n._init()):t.data(this,"masonry",new t.Mason(e,this))});return this}})(window,jQuery);


$('.inner').masonry({
    itemSelector: '.inner div',
    columnWidth: 0,
    isAnimated: true,
    isFitWidth: true
});
</script>           
</body>
</html>

I've just embedded all the code here for ease, but of course, you'd want that script at the bottom in a separate file normally.

Question 2

Just change:

.inner {
   margin:0 auto;
   padding:10px;
   border:5px solid green;
   display:inline-block;
   text-align:center; // changed to center
}

jsfiddle