I've read that time to terminal velocity for a typical human body (matters b/c of the amount of drag / air friction) is 5.5 seconds, which is almost exactly one DnD round (6 seconds). I know the rules for flying creatures who are suddenly unable to fly is 150 feet the first round, and 300 feet per round after that, reflecting that things are constant after 6 seconds (although I'm not sure if this is 'special' for previously-flying creatures vs suddenly falling).<br/><br/>Gravity acceleration is 32 ft / second / second, so rounding to 30 (a nice DnD number distance number), that would be 30 + 60 + 90...in the first few seconds, which adds up to more than 150, but that is ignoring air resistance.<br/><br/>You're quite right that falling distance damage ignores this (probably b/c terminal velocity is supposed to be fatal anyway for most people), so someone could perhaps use a maximum of 15d6 as that corresponds to 1 round of the aforementioned DnD falling, which happens to roughly be the distance at which terminal velocity is achieved.<br/><br/>Interestingly, since velocity is linear in time (if there were no air friction),  and thus so is momentum at impact, it's intuitive that the damage taken is linear in the distance fallen (although HP are an abstract, nonphysical concept anyway, so it wouldn't necessarily be 'unrealistic' if it weren't, as long as it's monotone) - so while the falling damage rules were probably just to be easy (1d6 / 10') they sort of make sense (if they are capped at terminal velocity).<br/>