Anyone signing up today needs this. It changes at midnight, and
yesterday's still works — so a code handed out at 11pm doesn't die at 12.
Add player
Leave the rating blank to start at 1200. Set it only if you're
carrying over a known rating from elsewhere.
All players 0
Data
Everything lives in this browser only. Export regularly — clearing site data wipes it.
Rating settings
K is how far one game can move a rating. New players use the larger K so they
find their level fast, then settle onto the smaller one. Changing these
recalculates every rating from the full match history.