By getting experience. Experience includes bad decisions you learn from.

Seriously, there is no magic rule. Just find the bike that seems to fit you based on what you already know and the advice of others that seems plausible. Road bike fit as you perceive it, is going to change as your body gets more used to the new things you are doing with it. How hard you intend to ride your road bike is going to have a lot to do with how any advice giving will help or be adverse to your perception of the fit.

If you are willing to change the things out or have them adjusted as your body changes, then that bike that fit you when you bought it can be made to remain comfortable up to a point with some money thrown at it for what you don't know yourself.