So what will give you a "true" 750'?
I think that is quite a hard thing to do. Constantin's rope will not work becuase the rocket could be flying horizontal. Barametric pressure probably reads a lower pressure i.e. higher altitude than true because of the venturi effects PeterH pointed out.
Knowing acceleration and attitude you might be able to calculate altitude?
Perhaps a laser rangefinder?
To get a true reading within 10ft sounds difficult to me.
I was using a handheld GPS last night and my altitude was shifting around by a couple of meters for no obvious reason. That is why they use a standard device in competitions I think.