It's nice to be able to do the math, but hardly necessary. The quick and easy way is just to plug your recipe into Brewer's Friend...... When you go to the fermenter, measure your OG and volume. Brewer's Friend defaults to 75%, and gives you a projected OG based on that... adjust your volume to match the real volume, then adjust efficiency until your OG matches the OG displayed........ That's your efficiency for practical purposes. That measures your mash efficiency....... unless you add sugars to your boil. If you are adding sugars, those sugars will be 100% efficient, and your mash may only be 75%, so you want to take a gravity reading pre-boil based on pre-boil volume and pre-boil gravity. If you want absolute grain to glass brewhouse efficiency you base your numbers on the volume that makes it to your glass........not fermenter volume.
The point is efficiency is different things to different people and if your object is to compare your mash efficiency from brew to brew...... do the pre-boil thing. If you want to have a "pissing match" with a fellow brewer, you need to be on the same page as far as what you are calling efficiency.
H.W.
