As your pack gets older, and deteriorates, your mAh capacity WILL decrease, therefore, as you fly, your voltage will decrease QUICKER.
80% of your pack's original stated capacity, or original measured capacity, for example on a 5000mAh pack, it will be 4000mAh, will NOT be 80% when it starts to deteriorate. Therefore, if you always spend 4000mAh, as the pack gets older, you are actually eating more than 80%, because the lipo's ACTUAL 100% is less than when you started. E.G. 4000mAh on a now 4500mAh pack is now 88.8%. The new 80% of 4500mAh is now 3600mAh.
When I fly my plane, I use EzOSD, I know my flight times, and I know my 80% mAh value. Then one day, as I was flying, I noticed my low voltage alarm flashing, and I thought it had to be a mistake, as I hadn't used near 1800mAh, of my 2200mAh packs. I was only at just over 1300mAh, and I wasn't near my usual flight time either. Had I been keeping a closer eye on my voltage reading, I'd have realised that something was wrong with my pack. So, from then on, I use mAh as a guide only, but always pay attention to the voltage. Luckily as it was only a foamie, there was no harm done, as I just glided in to land, as the controls were on a separate BEC.
For my MR, I always watch the voltage, as THAT'S what counts, when it comes to a pack failing. I too also use the cheap lipo alarms, just in case I lose a cell, on one of the parallel packs, this will let me know before its too late, as mAh readings on a parallel pack with a dropped cell is useless.
