its just dawned what your on about... i think we all had a similar discussion the other week?
with me its down to looking at what your gonna have that set and puttin just slightly over the right amount of water in so as to have a tiny little bit left over in a perfect world...
thats when you have to get it bang on, and its usually near as dammit, though id rather too much than not enough...
sometimes i get it wrong and have to mix a bit more...
still, if youve got everything prepped up (both sets, or maybe you roll 2 sets into 3 or more...) then i usually mix a bag and a half, even if say the ceiling only wants 3/4 of a bag the rest goes on something else, be it another 2 small walls, or just a reveal head, or maybe even stick a few more beads on next room...
whichever way you look at it, plaster costs 4 quid a bag, id sooner waste 4 quid than f'ck a set up running round mixing all the time when my time would be better spent on the end of the trowel..
my mate has a tall plasterers bucket and the line on it is clean below 1 bag and dirty above it so he just mixes 1 bag at a time in the bucket, very seldom does he mix small amounts...