So you're saying if you have crappy efficiency that batch just boil down to a smaller volume? No I would say that is not the best way, I want to know how much finished beer I will have. Plus you would have to adjust your hop additions if it was a significantly smaller batch. Better to dial in a decent efficiency so it is reproducible, then you can hit your numbers and predict your batch volume each time. It may take a few batches to dial in with good measurements but folks can help with that.
To answer your question to target a beer around 4.8% you'd want an OG in the 1.046-1.048 range post boil. If you are measuring pre boil you'd have to make sure it is a very well mixed sample which can be hard unless you're doing full volume mash. Then you would calculate how much volume you need to boil off to hit the target OG.