if you've ever heard of model railroaders named John Allen or E.L. Moore, they both used "imagineering". there's a point (and i do understand it exists at different places for each of us) where things cease to be measured and become more important to look correct. at 1/35 scale, the plausibility, or margin of error, for an ammo pouch is much different than a 1/6 scale figure. larger scales require more detail to look "right". tarps on the other hand, came in infinite sizes and may depend what you want covered. if it's bundles, a scale tarp may look monsterous unless you have scale thickness canvas from which to fabricate.
as far as your original question, i would use the trick of getting a picture of something you want to model, and then search the picture for something which you can readily dimension.in architecture, quite often it can be a common door size, or height of brick course which will ultimately yield the building dimensions. maybe if a soldier's finger would be four inches long, tucked in his ammo belt, the pouch may prove to be 1/5 longer, or 5".
i guess it ultimately depends where your pain threshold lies!