Exactly! The theoretical efficiency is fine- but each of us have different systems/grain crush/water chemistry.
One thing that really helps is calculating your actual efficiency and then seeing if it's the same over several brews. If it is, it's easy to predict probable OG for each time. Consistency is the important thing, not the actual %.
What I mean is this- with my setup I get 75% efficiency each and every time. That's perfect for me, as I can easily predict what I'm going to end up with. What's hard is when one time it's 60% and one time it's 80%- it makes it hard to hit an OG.
If you can look at your last few brews and the efficiency you actually got, then you can plan on it next time.
I hope that makes sense!
I use brewing software to help me out, because I hate the math. But either way is fine. If you want to post a couple of recipes and your OG, we can take a look and see if you're consistent or if we can give you a few pointers to be able to be consistent.