You will need 1 of 2 setups. Both will need an adj. PR length checker.
1. a checker spring, it replaces one of your valve springs so the spring pres. does not collapse the lifters.
2. Get/make a lifter "solid" and use this lifter with 3 or so seperate locations.
You need to think of the PR and rocker as related but 2 seperate items that work together but no the same.
The proper PR length is the length that fits the rocker arm setup that is correct in its geometry.
Shims are what you use to get the rocker arm setup to get the proper travel. The best rocker arm travel across the valve stem head. The travel should be the narrowest (least amount of travel) and as central as possible on the valve stem head. ***shims are not used to make a PR fit many people do this but throw off the rocker geom. in the end***
I did this by using a lifter "made solid". I used a dial and mag-base and broght the cam to its mid lift number. Mid lift is were the cam is at 50% of its lift. Take the lift and divide by 2. I took the measurement from the lifter so I made sure to remove the rocker ratio from the cam lift number to get the correct number at the lifter.
With the lifter at mid lift I put a little dab of grease on he valve head and installed the rocker. I had the adj. PR in place and brought it snug with the rocker so there was no play up/down in the PR. Another way to put it is I had slight drag on the PR as I rotated it (without making it larger/smaller).
I then removed the rocker and checked the patt.the rocker left in the grease on the valve stem head. If it was off center one way or the other I used a shim or removed a shim. It only took 2-3 tries and one of those was with a shim setup I knew would be way off so I knew what NOT to look for.
When the rocker travel at mid lift is a narrow line in the center of the valve stem head. The adj. PR is then at the length needed. There is one thing you will need to add...the amount of lifter pre-load as I used the lifer "made" solid so there was no pre-load. Just add that number to the length of the adj. PR when you have it measured and you should be golden.