From the website 
The gas spring lift consists of three measurements:
Extended Length is the overall length of the lifter with the rod out
Compressed Length is the minimum length with the rod pushed into the lifter
Rated Lift is how much the lifter will 'push'
A 15" Extended length = 9.5" Compressed + 5.5" of lifting travel (Stroke).The simple way to do this is by trial and error with a fish scale. You need to see how much lift is needed at the point where you will attach the lift to the door / hatch / cover. Remember, you will still want to have about 510 pounds of force on the place where you lift. You will also need to calculate the stroke and extended length of the gas lift.
For those who want to get technical, here is the long math to calculate this.
if you define: W = weight of door (in lbs) L = length of door (in inches) a = angle door makes with base at hinge (initially 0 deg, 90 deg when vertical) b = angle piston makes with base (initially 0 deg, varies with a) H = hand force applied at lip of door, and perpendicular to door Fp = force in ONE piston (assuming there are two pistons, one each side of door)  double the Fp for a single piston. A = distance from hinge to attachment of piston to door B = distance from hinge to attachment of piston to base Lp = length of piston (initially BA for the setup you show) Sp = stroke of piston from initial position (initially zero) then: b = arctan(A*sin(a)/(BA*cos(a))) Lp = (BA*cos(a)) / cos(b) Sp = Lp – (BA) and the big one: H*L + 2*Fp*sin(b)*A*cos(a) + 2*Fp*cos(b)*A*sin(a) = 0.5*W*L*cos(a) now, for a given A and B you can get angle b for any angle a for a given A and B you can get the stroke of the piston Sp for any angle a for a given Fp you can get H at any angle a, or for a given H you can get Fp for any angle.
Well, that was easy. WTH?
