My approach to a 80m2 ceiling.
1 and a bit bags stiffish caulk out all tapered edge joints then gauge a 3 bag bucket whichll first coat 50+m2 .
knock another 3 up slightly wetter speedskim the first 50m2 now scratch the other 30m2 on speedskim that.
whats left out of the 2nd bucket will almost lay the first 50m2 in knock 1 bag up speedskim what was just 2nd coated now lay the rest in which will be hanging fairly wash out then speedskim the lot.
This is obviously for a ceiling thats a sensible height and shape with straightforward access if any of these things get more difficult, either A the number of mtrs going on will come down or B the number of trowels needed will go up.