Help with Refractometer

When using a refractometer and taking first runnings during the sparge, how do you determine likely efficiency? For example, do you collect 3 gallons and then take the reading and then gross it up to 5 gallons? I could not find this in Palmer's book and hope the forum can help

you don't normally take the OG from the first runnings, but from a heavily mixed solution of the first runnings and sparge water... i.e. at the full 5-6 gallons pre boil
then do another post boil before pitching. Am I misunderstanding something or does that make sense? i.e. the first runnings are going to be really high gravity, but adding the second round will significantly reduce the gravity... something like for a 1.050 beer, the first runnings may be like 1.070 and second 1.030 and they merge to 1.050... (making those numbers off, so don't think they're anywhere close to real, just an example)

or people will use the refractometer to watch final runnings in a fly sparge, as going too low a gravity on those runnings can start to put you in a tannin extraction range (i.e. not enough sugar in solution and the mash pH is starting to rise)

I take it after I'm done sparging and bringing the wort to a boil...if you have an ATC refracto its no problem. However that is a way point for me to get an idea of where I'm at at that point in the process. I take a final sample at flame out for my OG reading.

Also +1 to malkore...use it to make sure you don't over sparge