...seems to be within 1 or 2 pitch values of the tuning that FamiTracker is using.
(At the risk of theadjacking) What do you mean by "pitch values"? You mean 1 value of the Pxx command or ...? Come to think of it, I don't really know how P81 differs from P80 in real terms like Hz.
Seems like it would be useful if FT had more pitch support, for writing well tempered music or whatever.
The pitch at the hardware level is determined by an 11-bit number which divides the CPU clock frequency. The pitch table is a set of these 11-bit numbers, one for each note in each octave.
P81 adds 1 to the pitch table value. P7F subtracts one. It's as precise as the hardware is.
So, if you know your pitch table, you could actually calculate a corresponding table of Pxx values to apply to each note for whatever desired temperament. (I've been planning to write some microtonal nesmusic for a while, but haven't gotten around to it yet.)
