89GTstang

I hope someone out there can help me!!! I have an 89 GT Mustang that I put a set of "Powerheads" on it. They are stock heads that have been ported and polished with larger valves. I am putting on roller tip rockers and it said to set the pre-load between 0.20 and 0.60 thousands. I got a pack of shims along with the rockers (16 thin and 16 thick shims). What is the easiest way to set the pre-load on the rockers or do I even need to??
Thank!!!!!

Black Horse

are you sure they want you to pre load the rockers and not the springs?