I thought OG was post-boil but before pitching. But after reading How to Brew, chapt 12.5 it says this:
But, when it comes to the efficiency of the mash and lauter, we want to think in terms of the pre-boil gravity.
Palmer's Short Stout (target OG = 1.050)
Malts
6.5 lbs. of 2 Row
0.5 lb. of Chocolate Malt
0.5 lb. of Crystal 60
0.5 lb. of Dextrin Malt
0.5 lb. of Roast Barley
(8.5 lbs. total)
For our example batch, we will assume that 8.5 pounds of malt was mashed to produce 6 gallons of wort that yielded a gravity of 1.038. The brewer's total sugar extraction for this batch would be 6 gallons multiplied by 38 points/gallon = 230 points. Dividing the total points by the pounds of malt gives us our mash extraction in points/pound e.g. 230/8.5 = 27 ppg. This value is good, if not great; 30 ppg is basically what everyone shoots for. Comparing these numbers to lager malt's 37 ppg maximum gives us a good approximation of our mash efficiency: 27/37 = 73%, while 30/37 = 81%.
But, when it comes to the efficiency of the mash and lauter, we want to think in terms of the pre-boil gravity.
Palmer's Short Stout (target OG = 1.050)
Malts
6.5 lbs. of 2 Row
0.5 lb. of Chocolate Malt
0.5 lb. of Crystal 60
0.5 lb. of Dextrin Malt
0.5 lb. of Roast Barley
(8.5 lbs. total)
For our example batch, we will assume that 8.5 pounds of malt was mashed to produce 6 gallons of wort that yielded a gravity of 1.038. The brewer's total sugar extraction for this batch would be 6 gallons multiplied by 38 points/gallon = 230 points. Dividing the total points by the pounds of malt gives us our mash extraction in points/pound e.g. 230/8.5 = 27 ppg. This value is good, if not great; 30 ppg is basically what everyone shoots for. Comparing these numbers to lager malt's 37 ppg maximum gives us a good approximation of our mash efficiency: 27/37 = 73%, while 30/37 = 81%.