My bad... that reply was to another thread!
The Diamond/Austrian/Canadian/European use of "mass" is more "correct" than the typical American usage of "weight" (an airplane's weight changes with altitude, but it's mass remains constant - fuel burn not considered)
Wouldn't the use of "weight" actually be more correct? Full disclosure: I'm Canadian! haha
As altitude increases, weight decreases and therefore the required lift would also decrease! If you were to be looking for a way to accurately calculate performance figures, weight would be the most correct I believe.