That's not the way to do it. Measure the
actual volume, make sure that it and the
actual OG are correct, and then you can believe the mash efficiency number. i.e. the mash efficiency number that results from entering the other two numbers, or the mash efficiency number that you have to enter along with one of the other numbers to get the third. All three are interrelated, and which two you enter and which one is the "answer" depends on what calculator you're using (and whether it's making a prediction or calculating an actual). But bottom line... if you have to fudge an already known (i.e. measured) number to get the answer you were expecting, that's not going to be right.
I think you're thinking that because it allows you to see the mash efficiency as what it needed to be to hit 1.050. But again, that's without using the actual volume, i.e. the one you saw with your own eyes before you rethought it.
I recommend reading the Mash Efficiency and Brewhouse Efficiency presentation available here:
http://sonsofalchemy.org/library/