20W at 0.8 Ohms ? That is 5V at 6.25A. Try aiming for 3 to 3.5V for a start. That works to about 11W to 15.3W. That should give you something decent. Too high a voltage across is going to fry the coil and cotton too fast (which can be further tuned with wicking and such, but we will come to that later). For now, start low first...
And since you are into rebuilding, aim for lower resistance like about 0.4 to 0.5 ohms for a good start. Then that should be able to push to higher wattages.
And you might want to give this a read:
OhmG! Ohm’s Law Explained for Vapers - EcigaretteReviewed
Sent from vapour clouds above