The amount of clearance as the valve opens (assuming no other variables - and there are big diffs between engines) is a function of valve angle.
The tightest clearance (fastest approach to the dome, needs the greatest absolute vertical safety clearance) is a vertical valve (such as GMC, Buick L8, BMC Mini) since the lift is the actual distance.
As the valve stem axis is tilted away from vertical, the distance is reduced, and is easily calculated by multiplying the lift by the cosine of the stem angle.
If the engine has a high stem angle, such as the Chrysler hemi at 26.5°, the distance is substantially reduced: .200" lift (a common figure for lift during overlap) × cosine 26.5 or .895 = .179", the actual distance the valve moves down toward the piston. An even more severe angle such as the H-D Sportster at 45° moves even less, only 70.1% of .200", or .141".
Obviously, a valve tilted sideways (such as "T" head Marmon, Stutz) does not approach the piston at all: the cosine of 90° is 0.
A 9° stem angle has a vertical component of 98.8% of the lift, or .198" for .200" lift.
However, the valve's motion out of the vertical plane (the difference between the actual lift and the cosine-derived vertical distance) isn't lost, it must still be provided with a clear path but it's across the dome rather than down.
The sideways scrub across the dome is the width of the clearance notch in addition to the actual valve head diameter (or whatever part of it needs more room). Since it also deepens as it approaches the far end, the relief notch shape is 3-dimensional and weird, but it's what you need if you have a big compression dome and must keep the chamber volume small for CR.
This calculation is even smaller for any angle less than 45°. The scrub for a 9° stem angle is only 15.6%, or .031" for .200" lift. Even the Chrysler only moves .089" for .200" lift.
The amount of clearance needed varies almost inversely proportionate to the stroke length: short strokes get close at very small lifts.
it's also affected by the rod ratio: a short rod moves away from TDC quickly in both directions, and needs slightly less clearance.
