Thanks, gents...
Nowhere to put a 60-tooth wheel on it.... The only possible location is where I'm planning to put a synchronizing cam when I do a single-tooth clutch for leadscrew reverse (the coarse threads, below 16 tpi, are driven by the spindle pulley before the 8:1 backgear, so aren't synchronous with the spindle, hence I can't just use the threading drive as it stands or I'd end up with lots of 8-start threads...) - the 70-tooth is on the spindle to drive the threads and feeds, so changing it isn't an option... Makes it difficult to add headstock dividing, too! That'll have to hang off the back of the spindle...
My thinking is that the microcontroller counting pulses / minute is really measuring Hz at the input, the 60 tooth disc multiplies to give revs / min, so by making the microcontroller clock 7/6 x the original frequency (28 rather than 24MHz) it'll count for 6/7 as much time so taking 1rpm and a 1 second count it would see 60 pulses from a 60 disc, reduce that to 6/7ths of a second it'll see 60 pulses from a 70...
If the microcontroller clock wouldn't take a 28MHz crystal it'd be a non-starter... But it looks like the "frequency counter" is direct from the chip manufacturer's data book 'sample circuits' pages, just with the type number sanded off ;)
It's not a PIC, it's a general purpose microcontroller, based on the 8051 (max 32MHz clock), so all its timing will be based on counting clock cycles with a DJNZ instruction or three... Totally clock-dependent.
Dave H. (the other one)