Scaling, The biggest subject on 405th. I would recommend searching out the scaling methods of Xtreme Tactics his tutorials are pretty good. and most of my scaling is near perfect when you follow those methods... However!
Using someone elses scale isn't the best way to do this because they usually scale the armor to their body adn not everyone's body is in the same proportions. So it's recommended that you scale it yourself for the best fit.
Next Space Marines are not really scalable to Humans for many reasons. But the long story short is the models and reference photos you will find for marines are never drawn to proportion So trying to scale them is nearly impossible.
Space marine are alot of trial and error but in anycase you have to make it fit yourself inorder to make it look good on you.