Struts, how to choose them for hatches?

[Salter]

How do you choose the right length and strength of hatch struts? Most of my hatches are fairly small battery, etc but a couple are big for fuel tank, etc. I guess 10 lb strength, but am kind of lost on what length to use. Is there a rule of thumb?

 

[airshot]

There is a formula for calculating the length. Upgraded some on hatches in my old motorhome. Dont remember the formula, but if you look up the strut mfgrs, they have the formula on there sites.