The scope height thing for HFT matters because the bigger the distance of the scope from barrel to scope centre, the more distance the pellet has to overcome before it hits it's zenith (top of trajectory) in relation to the scope... which means you have more drop on closer shots, which makes them more critical to rangefind for HFT, especially with reducers.
For FT, beyond the fact that a head up is a more comfortable position, the opposite is true... the high mounts allow you to see down the fall of the shot more past the zenith and reduces the amount of clicks needed. That said, I have a couple of rigs with varying scope hieghts and it perhaps takes 1/2 moa off every 5 yards.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep