Question for Stock Build Veterans/Enthusiasts

I have been interested in making stock builds that are visually and dimensionally accurate but using stock parts. My question is, how do I "scale" performance to fit the stock map? What I mean is, if for ex. I want to build a stock Atlas V (or any other current craft based on real craft) and I want it to perform as close to real life in terms of ability to lift realistic (relative realism based on weight/size) payloads to LEO/GEO, without over-performing (or under-performing), how would I go about scaling performance?

If for ex. I build an Atlas V of similar size and performance to the real thing, in general it would over-perform in Droo, so in order for this rocket to behave similarly as it would on Earth, I would have to scale performance by either making it heavier or dialing down the engine performance, but the question is how do I figure out how much? I am thinking of scaling down engine performance by the same ratio of the diameter of Droo to Earth, but do I need to take anything else into account?