I recently built a project requiring I lift a 100lb door 90 degrees, but I wanted it to stay open at the top, stay closed at the bottom, and hold position through most of the in between. I took some measurements, did the math, and got reasonably close, but the truth is I had to move the mounting points around a bit before I entirely got what I wanted.
As long as the strut is strong enough, you can move the mount points around to reduce the lift force to what you want, within reason. You just end up using less of the total stroke. In general you want the longest stroke and the lowest pressure possible, otherwise you will end up putting a lot of strain on the hinges. Also you can snap the door up too fast, and the momentum of the door at the top of the travel can damage the gas spring.
If it helps, I found a calculator here, but I can't vouch for it because it is not the calculation technique that I used.
http://www.enidine.com/pdffiles/Gas_Springs.pdf
They also make gas springs that are adjustable, and by that I mean that you can reduce the pressure from factory origional, but you cannot pump it back up, so you need to be careful and bring the pressure down in small increments. That won't fix poor geometry though. You can still end up with a door that won't hold open and won't stay closed if you get the geometry wrong.
I bought mine through Maxum Hardware and the adjustable force ones are not cheap.
Ameritool Adjustable Gas Springs - Maxum Hardware
This is the web page for the one I used:
Replacement Gas Springs Per Pair - Maxum Hardware
In particular, I used the GS26-8NN-150 at 150 lbs each and $29.95 a pair. You will probably need something closer to 55 lbs if you use 2 per door, or 100 lbs if only one per door. That assumes an effective stroke of 8 to 10.78 inches, but of course you don't have to use (don't want to use) the entire stroke that is available.
If you use only one spring per door then the door needs to be strong enough in torsion to take it, and the hinges need to be strong enough to not bend, especially when pushing the door closed. I prefer a 2 spring design for anything big and heavy.
If you want to get a feel for things without doing all the heavy math then find a spring scale (fish scale) and pull the door open from your hypothetical mount point. Just be sure to pull in the same direction that the gas spring will be pushing, which implies you know the hypothetical mout point for the gas spring on the other end. Be warned though, the forces, even in your application, will be quite high.
One other warning... Since the forces are quite high, make sure the mounting balls are bolted into some solid structure, like 1/8" thick steel, otherwise they will flex and put a side load on the gas strut leading to premature failure.
Good Luck
