jd 70 head

What block do you have in there? Is it straight bored or does it have taper like on a PowerBlock?
What is the bore and stroke of the motor? Do you have the head off? What pistons are in there? Are they completely flat or do they have cutouts in them?

1. You need the displacement of one cylinder =3.1415926*stroke*((bore*0.5)^2)
2. The cubic inches of the head (use water to measure)
3. The cubic inches of the block with the piston at TDC (use water to measure)
4. Add #1+#2+#3 and divide by #2+#3 - That's your current compression ratio.
5. So to see how much you will change the compression ratio, use the formula from #1 on the head to calculate what you can change it. Until you get to the eyebrow it will be fairly accurate. Subtract what you want to remove from your old #2 and recalculate #3.

This will just give you geometric compression ratio, actual PSI numbers will be relative but different. I don't have a good way to calculate that, some people estimate 20 psi per compression ratio but that depends more on flow in the engine.