Just did an interesting calculation. Assuming I DON'T or CAN'T drain the torque converter and also assuming that there is a total of eight litres in the transmission (prob nearer 6.5) and that I can drain three litres out of the pan, I have calculated that after four changes of 3 litres (ie drain 3 litres, fill 3 litres, drive and mix old fluid with new) I would have 57.5 parts new fluid and 6.5 parts old fluid ie I would have changed 90% of the fluid.