There are ways to determine that but it would take a mechanical or structural engineer to figure it. I think if you figured 1/2 or 3/4 of the total you would be safe. I used gas struts on anothere trailer I modified and just took an educated guess and it worked out great. It is better to have too much than not enough. Think about wind, rain, snow loads also. I would also add 5% for welds, fasteners, caulk, etc . 1x2 .072 is 1.83#/ft 14ga is 1.6#/ft. .040 ali is .564/sf. Russ