Frank,
It seems to me that changing push rod length can satisfy one or the other
criterion, but not both at the same time; either you get maximum extension at
half lift (the 90 degree angle point), or you center the contact patch on the
center of the valve stem. If the rocker arm length equals the stud/valve stem
separation, then everything is fine; otherwise, some compromise will be
needed.
Here's some instructions on how to pick the push rod length I found, which I
believe is what you described:
"It is desirable to have the least possible amount of roller movement across
the valve tip as possible. This will provide maximum valve lift and minimum
wear and side loading of the valves and guides.
To determine the roller movement across the valve tip, known as the "rocker
strike" or "contact patch". Apply a thin smear of bearing blue (Prussian blue)
to the valve tip, fit rocker arm and set lash clearance. Engines fitted with
hydraulic lifter cams set clearance to zero.
Rotate the engine by hand at least on full turn watching the roller movement
across the valve, the roller should move forward across the tip until half
lift and them back to its starting position at full lift.
The 'rocker strike' or 'contact patch' should be central on the valve unless
non standard length valves are fitted due to the intersecting angles of the
stud and valve centerlines.
Geometry can be changed by fitting different length pushrods or valve lash
caps, shorter pushrods or lash caps will bring the starting point further back
from the center of the valve, longer pushrods will have the opposite effect.
Ideal geometry will have a strike area of approximately .060" [1.5mm] wide,
central on the valve and have the major amount of roller movement at low lift
where the spring pressure is least.
Before running the engine it is vital to ensure free operation and at lease
.060" [1.5mm] of clearance between arm and valve spring, retainer, base of
rocker stud and back of posilok nut.
Clearance should be checked at both closed and fully open valve positions."
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