Canopies take between 2 and 5 seconds to open. At 2 seconds, that is a deceleration of 113 mph or a quite painful 5.5Gs. Five seconds is better at 1.1G
How long do you need to be under canopy to land safely?
Add 5 seconds for the canopy to make sure it is going to open - you don't want to be, as my grandfather called it, "jumping to a conclusion."
Decide how long you want to be in complete freefall for.
Add the 12 seconds from take-off to freefall.
The number of feet you need to be above your landing area is...
Acceleration + freefall seconds (s)+ canopy + landing seconds (a) = 750 + 183s + 503 + 18a
You need to decide what S and A are. If your chute opens in less than 5 seconds, then the 503 will be smaller. It is the integral of 183-([183-18]/5)x from x=0 to x=5. Change the fives into smaller numbers and the distance will be smaller while the G load will be bigger.