John, this is the formula based on my Imperial 280 but assuming its got a metric leadscrew, the head and gearbox should be the same on both its just the leadscrew that would be different.
We know the leadscrew will move the carrage 3mm per revolution of the chuck but we want it to only move by 1.75mm per rev so 1.75/3.00 = 0.583333
Now to get the same number with the gears
Basically divide the driver by the driven so the 40T spindle drives the 50T, 50T drives the 80T, 70T drives the 60T, this gives us the fractions 40/50, 50/80 & 70/60 noe expressed as decimals 0.8, 0.625 & 1.16666.
The gearbox ratios are A=1:1, B=2:1 & C=1:2 which give the decimals 1, 2 & 0.5.
You now multipy these decimals so 0.8 x 0.625 x 1.16666 x 1 = 0.583333
If the two numbers are the same you have got it right. In our case they both come out at 0.583333

Edited By JasonB on 12/07/2013 08:16:54