Help with drive belt setup

I have created my own homemade CNC router. I have kind of a "hybrid" setup where my Z and X axes are screw driving while the Y is belt driven. Everything is up and running fine but I'm having a difficult time dialing in the Y axis (the one that is belt driven). I'm unsure of what values to use in the "Pulley teeth: (Motor Leadscrew)" field and the "Leadscrew Pitch" field. I currently have "Pulley teeth..." set to 1.0:1.0 and 0.635 in the "Leadscrew Pitch" field. I arrived at that number by measuring the diameter of the belt pulley, converting that to a circumference and then dividing by one. The result was really far off so I have tweaked the value little by little. So when I cut out a part, the dimensions in X and Z are spot on but the Y measured value is slightly larger than it should be...about 1/64 of an inch on a 4 inch part. Not bad but I would like to get it better. So my question is this....What is the correct method to mathematically arrive at the values I should enter?

UPDATE: So...I found the issue. Cheap router bits / adapter. I was using some cheap 1/8 bits and a 1/4 adapter. Got a high quality 1/4 spiral bit and everything works perfect now. I'm within +/-0.008" on a 6 inch part which is more than accurate enough for my needs. Lesson learned!

Oh and the correct way to calculate the value I needed appears to be: (# of Pulley teeth X tooth pitch) / 25.4 to convert to inches per revolution. Then 1 / the number you get above. That is the value to enter for Leadscrew Pitch.

So in my case (20 X 2) / 25.4 = 1.5748
1 / 1.5748 = .6350 (which just happened to be the value I worked out thru trail and error)

If I'm wrong about that someone please correct me!!

You actually guessed all my details...GT2 belt with 20 tooth pulley, 1600 steps

No sort of reduction is being used. I'm running in inches.