Not as sexy as viewport text maybe, but you can display a floater window that shows you the dimensions of any selected object. Utilities > Measure > New Floater.
Just mask and extract the details as floaters will do. You will get same surface as the helmet underneath. Or use that brush to conform/bend the floater to the subtool behind.
i would uvmap the highpoly, and render out an ao map without the floaters and then render out the aos for the floaters separately, and then bake the two like the normalmap to the lowpoly
To get rid of floater shadows you can do like PolyHertz suggested, only with one change to make things easier. You hide or get rid of the rest of the mesh when you render the floaters. Makes combining them much more simple, provided nothing casts a shadow on your floaters...
When modeling the magazin try to make it straight out and then just bend it in the end. So you are able to create this complex geometry easily. For the problem with the floaters. The geometry around the floater needs to face in the same direction als the geometry of the object below. So if you have a floater on a curved…