Replacement struts for shell: How man pounds of force?

I have a no-name (or I can't remember the brand) camper shell on my 01' Taco and the struts are gone. The window/door falls all the time when open.

I know I need 15'' struts but I'm not sure what pounds of force I need. Any thoughts?

I'm a musician and I often load my amp (half stack) and other gear into the bed so I'm often going into the bed and out and I want to insure the movement of me getting in and out won't cause the door to shut on my head (which it does now).

 

i got mine at auto zone...i just gave the guy at the counter the old one and he went back and found ones with the same ends and lengths. the new ones work very well and since the only ones they had in that size are for hood props they pull it shut at the bottom of the stroke as well as push it open and hold it open. no idea on pounds factory but by feel its the right strength- new says 19lbs.