Gets the coordinates of the mouse relative to a container.

getMouseCoords.js

var container = document.getElementById('container'), // Pick the element(s) you care about
    halfBoxSize = 0, // Facilitates placing boxes (click in the middle, get the upper-left coords)
    roundPxToNearest = 5, // Snap to grid
    roundPctToNearest = 0.0000001; // Precision for percentage values
container.addEventListener('click', reportLocation, false); // mousemove is another useful event

function reportLocation(event) {
    var floorCoords = this.getBoundingClientRect(),
        leftPx = roundPxToNearest * Math.round((event.clientX - floorCoords.left - halfBoxSize) / roundPxToNearest),
        topPx  = roundPxToNearest * Math.round((event.clientY - floorCoords.top  - halfBoxSize) / roundPxToNearest),
        left =  roundPctToNearest * Math.round((100 * (leftPx / floorCoords.width)) / roundPctToNearest),
        top  =  roundPctToNearest * Math.round((100 * (topPx / floorCoords.height)) / roundPctToNearest);
    console.log('left: ' + left + '%; top: ' + top + '%;');
}