How i did it.. there will be other ways i just made this up as i went along
the correct length is when you have the rocker at the mid point of its rock. when the cam has lifted the lifter by half of the total lift, and hopefully the roller tip is in the middle of the valve stem.
Vary the angle of the rocker to achieve this middle coverage of valve tip, slightly, if you need to, but check it doesn't go into bind or hit the retainer at the extremes.
just paint the valve tip with sharpie ink and roll the cam through a rotation or two to get a valve tip coverage pattern.
for solid flat tappet
i took a scrap pushrod which i already knew was too short because using it i could not achieve rocker level at mid lift and rocker roller was off on the valve tip, rocker was leaning too ar back for my liking onto the pushrod side at all lifts
installed my old pushrod so i could see where it was visible in the partly assembled engine and i could access an adjuster
removed it
cut it at that point
stuffed a piece of threaded rod down 1 end and left it so that it stuck out far enough to mount the other end onto it
i then used hardened steel washers to shim the length at my cut-point until it was right
ordered pushrods that were standard length + the thickness of my shim washers
done this way because there are no "checking" pushrods to be purchased for my engine. if you can get one it comes with an adjuster already and you just measure it or send it to the custom pushrod shop.
if you have rockers with a screw in adjuster cup,
you do this with the adjuster wound out by 2 threads only you don't want a 1/4 inch of thread showing, the adjuster will just snap in use, and you do need a little adjustment in both directions hence 1-2 threads out is your base setting
clay smith sorted me out with a great set of pushrods
used them because i know a man who brings stuff from clay smith to the UK
hydraulic and roller method may differ, saying that because i haven't done it
i had bit of a learning curve mind. ford 1.72:1 rockers, on a mopar engine, using chevy ball stud, studs and posi locks, to hold down individual trunnion mount double roller rockers on an engine where the valves have 18 degrees between them. i.e wrong geometry in any way caused the rocker trunnion of the inlet on one cylinder to hit and push out of alignment the exhaust rocker trunnion on the next. perfect example of a complete pain in the ***, but very obvious when you got it right.
Dave