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VALVE LASH or VALVE CLEARANCE is measured between the top of the valve stem and rocker, not the push rod.
If you run hydraulic lifters, they are always under "some" preload
If you run mechanical lifters, the "push rod" clearance is never actually measured. It would be somewhere in the vicinity of the valve clearance multiplied by the rocker ratio, I guess
In other words if the valve clearance is .025 (say) and you have 1.5 rocker ratio, the (in theory) clearance at the pushrod would be .025 / 1.5 or about .016