Mancini/HS 1.6 rocker not giving me the lift expected

I wouldn't use a checking pushrod with anything other than a checking spring. That's the first thing I'd look at.

Are you 100% sure you're on the base circle when you start your measurements, and also set preload?

What are you using to measure lift? Can you share a pic of your setup? I had to make a big to adequately attach a dial indicator on my heads. Mounts to the valve cover holes.