Thanks, that's good to know.
I've only read the procedure in journal articles, where that detail was never pointed out. I had assumed, since they were running off the wort through a filter paper, that they were using the volume as well as the gravity. I hate it when research papers leave such important details out of the method section.
I had to look for this too. It didn't make sense to me that the lauter efficiency would matter. And if it mattered why weren't there any specifications about the size of the filter paper, for example. The larger the filter paper, the larger the losses during lautering.