RE: how high is too low for a tree stand?
The shooters that have said; let the terrain dictate, is absolutley correct. There is no such thing as one size fits all when selecting stand height.
I have killed deer in stands that are 24" to 36" off the ground to 20 feet in the air. Even 20 feet can be too high if the shots that will be afforded will be close in. Too much angle reduces size of kill zones.
The buck I shot last season was from a stand that was 24" off the ground. Height was dictated by cover, natural fields of fire, and prevailing winds vs. direction of usual deer movement at a particular hour of day at a particular time in the season.