Roller Rockers 1.5 vs 1.6

For instance, a cam with .500 lift and .100 P/V clearance at TDC will never get closer than .100 regardless of cam timing.

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This just isn’t correct.
It’s all about the cam timing.
If it were an intake lobe that had .100” clearance at TDC, and you advanced the cam 10* it will then have noticeably less v/p clearance.
(Advancing the xe268 cam 10* would eat up .052” clearance with a 1.5 rocker)
And the exhaust clearance would grow.