Transmission Ratio calculation help

I have an AXYZ router I am retrofitting, Working on Pulse per inch calculations, I found info that is close but not sure how the math works

Paperwork on a similar model says...

Transmission Gear Ratio

24 Tooth Pinion Effective Diameter: 1.2 inches (30.48 mm)
Main Pulley: 60 tooth
Minor Pulley: 18 tooth

Transmission Ratio: .88422 REV/INCH / .3481 REV/CM

(I believe this works out to 3.33 : 1 Ratio)

My question is what is the formula to get the Rev/Inch number calculated to be .88422 ?

Pinion Gear / Track is 20 DP/Module 20 Pressure Angle

In my case I actually have
24 Tooth Pinion Effective Diameter: 1.2 inches (30.48 mm)
Main Pulley: 60 tooth
Minor Pulley: 24 tooth

I believe this works out to 2.5 : 1 Ratio

What is the Calculated Transmission Ratio with the 24 tooth Minor Pulley vs the 18 Tooth Minor Pulley

Trying to calculate my Exact steps per inch, 3183 Pulses per Inch is really close but not sure its exactly right
Servo Driver is set to 3000 Steps per unit

Any other info needed?

Thank You
Ed