Need help getting UV Unwrap to 1:1 scale

I often have to unwrap my models to apply textures that properly wrap around their edges. After unwrapping the mesh, I use UV Editor and drag a rectangle to an arbitrary size and adjust the placement of the unwrapped meshes within. The problem I have is that the unwrapped meshes are never to the correct scale. It’s evident when I then import the model into my rendering software (KeyShot), apply a texture, and set the mapping to UV. I always have to then scale the texture in KeyShot (usually to rather extreme amounts, generally ranging from 0.003 to 0.005) to get it to appear the way it should. I try to match the size of the mesh object to the size of the UV unwrapped mesh in the UV Editor, but it does no good.

Hello - can you post an simple example file?