One of us is doing the math wrong. I too would guess 0.010" to 0.020"

0.020" drop of both valves is almost 2 cc. If my math is wrong, please let me know.

Surface area of the entire valve verses the notch is not a good comparison.

My point is that if someone is getting 84 cc on a set of 915s, it might be be that it has had one or two NAPA valve grinds. Maybe just factory variability...I don't know