I've run testbeds with 100 and 1000 mines (eff 10). My results are as follows:
original formula is accurate for conc >= 30 and conc <= 100.
proper constant for 100 < conc is 125 kT (minor change).
proper constant for 4 < conc < 30 is: 480 kT/level
levels 2, 3, and 4 are 1,200 kT/level
Here is the updated formula:
con | minerals/level depletion
100+ | 125 kT
30-100 | 12,500kT * ln(n / (n - 1))
5-29 | 480 kT
2-4 | 1,200 kT
1 | infinite
It appears to be accurate to +/- 100 kT for depletion from 100 conc to 1 using large numbers of mines (at least 100).