An easy way to eyeball it and get an idea is to observe the angle of the screw relative to the pushrod. If geometry is close to optimum, the angle at 0 lift should be exactly the same but opposite at full lift.
Another way, if you have a dial indicator, position the rocker arm so the valve is at half lift. At this point, the screw and pushrod should be directly in line.
The best way is to blue the tip of the valves, turn the engine a few times and then observe the pattern on the tip of the valve. Add shims until that pattern is the narrowest. This is best to do with an adjustable pushrod so that the adjuster screw remains in the same position, and therefore the rocker assembly has the same ratio for every test. If you simply add shims and then adjust the screw, you change the ratio each time. You will probably run out of adjustment, at some point as well. Once the shim thickness is known, make spacer to match and order pushrods of the length determined by the adjustable pushrod.
Lawrenceville, GA