Custom pushrods, how much stock lifter preload?

I'm running a 5.9L Magnum short block with Edelbrock RPM heads for an LA engine with a custom-grind Racer Brown hydraulic roller cam. I'm using the stock roller lifters which the cam grinder (Jim) gave the OK. Initially I'll be using factory stamped LA shaft rockers but with .548" max lift on a fairly steep roller profile I'll be giving them close attention until I can afford adjustable roller rockers. Anyway, mocking things up with the heads torqued down on their gaskets the stock Magnum pushrods are way too long, holding the valves open to where they're touching the pistons... After I figure out length with a pushrod-checking tool with no compression of the hydraulic lifter, how much preload should I add to that figure?