How the payment is derived
Base points are fu × 2^(2+han), capped at 2000. A non-dealer ron collects four times base, a dealer ron six times, both rounded up to the nearest hundred. On a self-draw a non-dealer takes double base from the dealer and single from each other player; a dealer self-draw takes double from all three.
From five han up, fu stops mattering and fixed values take over: mangan at five, haneman at six or seven, baiman at eight to ten, sanbaiman at eleven or twelve and yakuman from thirteen. That is why the fu field disappears once you go past four han.
Each honba adds 300 on a ron or 100 from each player on a tsumo. Riichi sticks are 1000 apiece and the winner sweeps every stick on the table. At an exhaustive draw the 3000-point tenpai payment splits by how many players are tenpai — one takes 3000, two take 1500 each, three take 1000 each.
Dealer repeats and final settlement
The dealer repeats and honba increases when the dealer wins, otherwise the deal passes; at a draw it depends on whether the dealer was tenpai. All of that is derived from the history, so you never have to remember who is dealing or how many honba you are on.
When the hanchan ends, open the final settlement card. It converts raw points against the return score, applies rank uma, hands the oka to the winner and turns the result into money. The defaults are 25000 start, 30000 return and 10-20 uma.
Common questions
I am bad at counting fu
Fu only matters at four han or below, and picking 30 fu covers most hands. Pinfu tsumo is 20, chiitoitsu is 25, and an open hand with nothing special is 30. Above four han fu is irrelevant.
Does it handle three-player mahjong?
This is four-player riichi. Sanma splits payments differently, so for now use the plain score pad and enter the point movements by hand.