I presume you are putting the heads on a 305 block, right?
IMO you need to CC the heads and work out the combustion chamber volume. Then you can accurately compute the real compression ratio that you have.
Next, you can put the heads on the engine with a gasket, temporarily, and use a tiny amount of putty on top of the piston, and carefully turn the engine over by hand. The valves will squish the putty to show you the actual clearance you have right now.
Using the info gathered you can calculate the amount of material you can safely remove to achieve the desired compression ratio. IMHO 10:1 is the absolute upper limit.
But IMO proper porting, a 3-angle valve job, gasket matching etc should have priority.
You want to avoid a parts mismatch.
I once bought an engine not knowing the parts combo. I think it was 305 heads on a 350, (not sure)..... anyways the parts mismatch meant I had sky-high compression ratio with the poor flow of 305 heads....... so I had a low-power engine that needed premium gas with an octane booster..... a disappointing combo, because it was slower than a stock 350 but more expensive to operate.