What's the ideal preload measurement for a 1968 455. Comp Cam OEM replacement rockers and lifters. A mild cam has been added as well. I'm getting .0269" currently but I'm not sure if it should be a little higher.

 

I think up to about .050" is OK. The hydraulic lifters will adjust and take up the slack to about that amount, so at 0.027" you are within the acceptable range.

 

Comp calls out .030-.040 (1/2-3/4), I think your .027 is probably fine.