If there is a factory slug that runs in the 1,500 to 1,600 fps range, and weighs in the neighbourhood of 320 to 380gr., that would be a good one to regulate for. It will make working up your own loads with balls or slugs easier, I'd guess.
Considering in 'bore' rifles, 20 is a small bore, perhaps some sort of heavier than ball slug will be best. Balls run in the 320gr. range.