I have a heuristic - a thousand rounds for each gun caliber as a reserve.
My thoughts are similar, although I set my comfort levels depending on number of guns owned in each caliber, frequency of use and whether or not I reload the caliber and can make more as needed.
very high use and no reload - 7.62x39 and .22LR (2,000 or more each)
high use (reload) - .38 special and 9mm (1,000 each)
medium use (reload) - .45 ACP and .380 ACP (500 each)
low use (reload) - .44 special, .44 mag, .357 mag, .25 ACP (100-200 each)