This is very geeky but someone else out there might appreciate this info. I was just analysing an AY sound chip emulator (the libayemu one) and trying to work out how its "tone" control relates to the actual note that is produced. It seems to be an inverse relationship, something like this:
frequency = (109300 / tone) + 3.70727 tone = 109300 / (frequency - 3.70727)
It's not a perfect approximation but it works pretty well.