The "Problem" with this ^^^ arrangement is that you are UNBALANCING your cells continuously! The upper cells have a higher potential to stack 0v, so there is more leakage current through those cells. Now if you have huge cells, and you use massive resistors (>1Meg) this can be minimised, but the setup you have their has significant leakage current, so if you have small Ah cells, you'll find you always have the top cell going flat first. This is REALLY important if you are using the cells for a device that has a long "on time" and a small current draw, ie you'll waste most of the energy in your cells!
What you need to have is a circuit that is entirely powered off the full cell series stack, so equal current is pulled from all the cells, and then only pulls a TINY amount of current, evenly from each cell during measurement. This is not a trivially thing to sort out, it rapidly gets very very complex. Hence i'd suggest just using a COTS cell monitoring IC as previously suggested. Plenty to choose from these days, and whilst those chips might look expensive in low volumes (can be as much as $10 per IC or more!) when you add up the BOM cost, and dev time to make a home-brewed solution work, nah, it's just about never worth it!