Cam lifter questions

[Bad70sbchevy]

I'm doing an engineering design project for school on designing a high rpm hydraulic roller lifter for a small block chevy. I was wondering what a lifter's seat height refers too? Is that the distance from the tip of the lifter where it contacts the cam to where the pushrod contact the top of it, or is it the overall lifter height? Also I was wondering what causes hydraulic lifters to lose power at high rpm. I always thought that it was because the lifter assembly collapses at high rpms, but I see that sometimes they will pump up at higher rpms. Is the later from not running high enough valve spring pressures? Just curious and thanks in advance.