Pre-boil gravity question

Greetings.

Partial mash home brewer here. Since I'm not doing a full boil (only about 4 gallons), is there even a point in taking a pre-boil gravity if I'm going to end up adding top off water post boil? I'd like to be able to calculate my efficiency. Will the top off water (and lack of extract) make it pointless? I know the top off water is added after the boil, but I'm just making sure.

Oh, and about extract efficiency. Is it pretty set in stone?

Well fellow, I'm an all-grain brewer, I cannot help you with the extract efficiency question, but, I'm sure that the top off water is added after the end of the boil. There are tools to help you calculate final boiling volumes, diluting gravities and that kind of stuff, I do recommend BeerSmith. Just remember to sterilize your water before put it on after all.

To measure efficiency I would use a pre-boil gravity, although I suppose you could also take a post-boil reading before a flameout addition of extract and top off water - either way you just need to use the right volume in your calculation. You should get 100% of the yield from extract so as long as you know your volumes you can always calculate things later rather than worrying about mixing issues.