This is a really complicated topic.
1st: "mid-lift" is where the valve stem and a line through the rocker pad and rocker pivot (shaft or ball) intersect at 90° @ 50% lift. This produces the largest valve curtain area vs. time combination of all variants, but this doesn't always produce more power. It also has the lowest side-thrust vector, but that doesn't mean the least wear, since the highest spring pressure occurs at the worst angle. It's only designed into some roller rocker arms, since Miller holds the patent. The other rollers cannot be shimmed, spaced etc. to get mid-lift - it's not an adjustment.
2nd: mid-lift is not built into any "pallet" (non-roller) rocker arm (such as almost all OEM). You can modify things, raise parts and adjust the valve side lever to get this angle (but only on one side), and it produces a substantial error on the pushrod side.
The original rockers are most frequently designed to place the pivot below the stem top at 1/2 lift, which reduces the scrubbing force for best wear.
What do you have, and what's the problem?
