How to select gas spring ?

This is how I did it in the past, not sure if it's correct, but it's how I did it. it was for a metal testing trunk with a lid that weighted ~200 lbs.

I drew a starting Free body diagram of the lid with a pin connection at the hinge. I then Calculated the Moment it would take to open the lid holding the handle at the end on the lid (36 inches).

M(Gravity) = Force*Distance to CL
M = 200 lbf * 18 inches = 3600 in-bls.
Open = 7200 = X lbf *36 inches = 100 blf up

I decide to mount the Gas spring in the center of the Lid which gave the moment arm length (R) of 18 inches, and calculated the force required to lift 80% of the Total weight (I still wanted some heft incase something popped inside). Since I'm going to use one on both sides, I need to consider both springs there for 2*M.Spring rather then 1

2*M.Spring = .80*M.weight
2*M.Spring = 2880 lbf-in
Force. Spring = (5760/R)/2 = 80 lbf

M.open + 2*M. Sping +M.wieght = 0
M.Open + 2800 - 3600 = 0
M.Open = 800
F.Open = 800/36 = 22 lbs.

I went to McMaster Carr so that I could look through a library of parts. I filtered the springs by Extension Force that match my requirements. With my model I new the Stroke Length required so I filtered out any springs shorter than that. I then choose the spring that was the shortest in that selection. I Down loaded the model to make sure it would fit, and mounted in to the best of my ability. I tried to make in Perpendicular to the lid when closed so that the force would fully go into the lifting the lid.

After that I Researched cheaper springs online since McMaster is notorious for being expensive/shipping cost biting you. I sent the part to our welder who whipped it up, and now it's some where in North Dakota Oil Patch... testing tools, or maybe just being a very expensive Tool Box

Anyway, that's what I did