Ah, thanks. That makes complete sense and I started off thinking that's what I was going to have to do, then I read the Wiki (text below) which suggests that
I assumed (always dangerous I know) that the steps/mm were based on the default 16x microstepping, and everything else was calculated from there. The Wiki appeared to confirm that assumption, but you always see what you want to see…
When RRF sees a M92 command it takes that as the number of microsteps/mm at the current microstepping rate. If you subsequently use M350 to change the microstepping, then the steps/mm is recalculated by the firmware to allow for the change. If you then issue another M92 then that will be the number of microsteps/mm at the new microstep setting.
So if you want to experiment with different microstepping, it's probably easiest to put your M92 command, calculated at x16 microstepping (the default at power up) before the M350 command. Then you can adjust the M350 command without having to adjust the M92 command.