I got into the 4 thousands one or 2 times... but I can't pinpoint what is it that makes some flights go higher than others... most of mine were around the 3500 mark

I did figure out a way to land every time without crashing the craft... it seems it's forward flight that induces damage on landing, so if you reverse the glider after booster separation, it will want to fly in the opposite direction, and will slow down considerably... your vertical speed doesn't matter, just try to hit the ground with as low horizontal speed as possible 

Awesome game!!! It got me wanting to finish my Kerbal Space Project idea...    (as if I didn't already have enough work in college)

Cheers