Assuming you won't have an end and start being the same and start<=currentPos<=end
var currentPercentile = (currentPos-start)/(end-start);
Update
if your end and start can be the same value then it can be 100, 0, or unknown since it'd be outside the range, your choice.
var currentPercentile = 100;
if(end-start >0){
currentPercentile = (currentPos-start)/(end-start);
}