I've had good success weighing the lid to find the appropriate gas springs. In this case I would block the bed platform in the open position and disconnect the struts at the attachments to the frame. Extend the struts fully, wrap a rag around the shafts to protect them and snap a pair of vice grips on them to hold them fully extended. Position a bathroom scale centered over the frame attachment points on each side and block the scales perpendicular to the struts. Put the free end of the struts on the center of the scales with a small board in between to spread the load and let the platform down to weigh it. If you only have one scale and can only weigh one side just divide the weight by two. Now you know the weight rating you need.