it's complicated to marriage speed with pixels only by watching
normally i count seconds in psp to dig and try to implement on mine
my lemmings walks little bit faster than psp,it's the way xna update all 60 times second
basher is quite similar than psp
climber i think is slow too
floater falls 3 pixels too,etc.
i do all first watching and counting time and all need more precision i know but i have troubles
because i don't know exactly when and how much to erase,build,move etc.