The inverse square rule applies to enlargement the same way it does to studio lighting - double the distance, lose a stop, halve the distance, gain a stop. So going up to a 16x20 from an 8x10 is roughly 4x the distance, or two stops. If your base exposure is 3 seconds at f5.6, then give it 12 seconds as a starting point on your test strip (still do test strips!). you'll probably want to go up at least one grade in filtration, maybe a grade and a half. So you'll have to add another half to whole stop for the filter grade increase too. Try starting at grade 4, 24 seconds, and give your test strips three exposures at 12, 24, 36 and 48 seconds to see where the truth lies.