Thanks for that reference, Bsharp. I tested it against my chronograph data from weight-corrected charges in three rifles. In a 36" barreled .58 flintlock Hawken and Swiss 3F, 12 tests of 45 shots averaged 1713 fps. The PMAX calculated 1765 fps for the same, 52 fps higher.
For four tests of 23 shots and Swiss 2F, my average was 1674 fps compared to 1631 fps for PMAX, 43 fps less. For 10 tests of 42 shots of Swiss 1 1/2F, my average was 1604 fps and the PMAX was 1649 fps, 45 fps higher.
In a 24" Leman flintlock, four shots of 100 grains of S2F averaged 1598 fps, PMAX predicted 1545 fps, 53 fps less. Then 100 grains of S1 1/2F averaged 1413 fps compared to the PMAX of 1464 fps, 51 fps more.
A 33" .54 Bridger averaged 1659 fps for four groups of 15 shots of S2F, PMAX said 1578 fps, 81 fps less. Ten shots of S 1 1/2F averaged 1804 fps, PMAX said 1738 fps, 66 fps less.