That is a confusing point. Some people think the roller diameter is important, others don't. The engineers at Chevy recommend the "low point" or 90 degrees at 2/3 lift, approach for roller rocker arms. I've always thought that the low point design should be used on race engines with heavy springs while the 90 degree at mid-lift works okay for production cars and mild performance engines.
Here is another confusing point: If you set the rocker shaft for "low point" while measuring from the center of the roller wheel, you are actually close to "mid lift" if you measure from the valve stem tip!!
If you want to open another can of worms, there is a debate on what part of the lift curve sees the most stress, hence valvetrain deflection. A clue: it's NOT at full lift where spring forces are highest!
