Something like this might be easier... unless you are dead-set on using span9
<div class="container">
<div class="row">
<div class="well span10 offset1"></div>
</div>
</div>
If you use span10 and offset it by 1, it will always be centered as it is a 12 grid system (1+10+1). Also, you should add a row
div under the container.
Or you could use span8
and offset2
...
If you are dead-set on span9 then you can add this CSS which will center it..
.well {
margin:0 auto;
float:none;
}