calculating efficiency

Just wondering when you calculate efficiency do you use your pre-boil volume or final boil volume?
my pre-boil volume is 5.75G but after I boil I'm aiming for 5. So not sure when I divide by 5.75 or 5?! I normally take a reading soon after mashing.

thanks in advance for everyone's help!

http://www.howtobrew.com/section2/chapter12-5.html/

I personally just pop my numbers into BeerSmith and it calculates everything for me.

You can do either - just use the volume that's there when you take the gravity reading. As a simple example, say you have:

10 lbs 37ppg grain
1 lb 34ppg grain

You can use the equation:

gravity = efficiency * (10*37 + 1*34)/volume

Where gravity here is the last two digits of the gravity reading (i.e. 1.0xx).

So say you get 75% efficiency, then if you measure the gravity pre-boil (with a volume of 5.75gal) you should get a gravity of 0.75*(404)/5.75 = 53

If you measure gravity post-boil (with a volume of 5gal) you should get 61 (1.061 on the hydrometer).