In fact there is not preload because they are solids.
Solid lifters DO NOT get preload, they get LASH.
The easiest way is put the appropriate feeler gauge between the valve stem and rocker tip and spin the pushrod as you tighten the adjuster screw. When the pushrod won't spin you're done. Tighten the nut.
If you put preload on a solid lifter the valves will never seat and the engine will run like CRAP - if it runs at all !!
