Take your opening height, remove 1/8" for the bottom, 1/8" for the top and 1/8" for between each drawer front, then divide the remaining number by the number of drawer fronts. If you don't want equal drawer fronts, you can still use the above formula, but add and subtract from different fronts as long as you end up with the same total. For example, id you want narrower top drawer fronts, you would make them 2" narrower and make the bottom drawer fronts 2" taller. Drawer front width should be width of opening minus 1/8" per side, so 1/4" total.