ARINC
The actual driftdown altitudes can be found in the QRH for given weights and temperatures. However, for the B744, a good 'rule of thumb' is to subtract the aircraft weight from 630,000 (3 engine) or 500,000 (2 engine). Hence at a weight of 300,000kgs the driftdown altitude on 3 is 33,000ft and on 2 is 20,000ft. (knock the 000 off if you work in tonnes and FL's)
Airclues