Pre-load is sometimes something that is mis used. However there is reason to use it on certain applications.
Standing at the back of the car, veiwing the rear axle, on launch the rear axle wants to lift the right wheel into the wheel well from the forces of torque. A high powered big block combination that makes say 650lbft of TQ probably rolls out around 4500lbft by the time it hits the tires due to the multiplication of the gears in the trans and axle. So in the case of an overpowered combination a small amount of pre-load can help manage this action. Pre-load is sometimes a bandaid fix for something out of whack. Excess pre-load can cause a dangerous situation at the finishline when the brakes are applied because the forces are opposite.
To answer your question specifically would require more input, but in most cases ladder bar cars need little if any pre-load. The farther apart the bars are the less needed. Same applies to HP, as more power maybe a lil pre-load to manage launch. We used to set our pre-load needs with the drivers side adjuster...make it longer and only turn the adjuster 1 flat on the nut at a time and test from there. Just cranking away by guessing or even using corner weights never really got it done. On my last combo I used 2 flats from neutral and launched straight as a string every pass, any track. That was a 477 BBC w/14.25:1 pistons, .714 roller, Dart/Merlins, and a Turbo 400 w/5500 stall and a brake...this was in a 72 Camaro with 10.5x29.5 tires and a 4.88 gear. At 3545# the car had a best 60' of 1.40 and a best run of 10.30 @132mph.
I hope this was helpful.