We're obviously on two completely different paths in thought here.

It's not the extension of the piston in between open and closed that I'm worried about. It's the extension when closed, which isn't necessarily fully compressed. When the lid is open, the spring will be at full extension unless I employ some other device to limit the lid opening. How compressed the spring is when closed depends totally on mounting position and that's what I don't see how to figure out without moving the lid.

The spring will actually only work in very few positions, irrespective of its force and the weight distribution of the lid. Speaking strictly of its ability to extend and compress, only very few positions for each end will allow the lid to get to the angle desired when open and flat when closed. Obviously the force is important to complete the functionality, but it's irrelevant if the lid can't be actuated through its desired path (from 0 to 65 in my case).

The idea behind me getting a compressible object into Sketchup was so that I could actuate the lid and check whether certain mounting locations would allow it to both close flat and open to the desired angle. That's why I mentioned being able to move the point which represents the most outward on the lid in ggb.