everyone starts with 1000
first game you play, if it you both had 1000 points, it would be 1000/1000 x 20 which is 1 x 20 which is 20 points which you win and they lose, you end up at 1020 and they're at 980
if you lose to someone with 4000 points it would be 1000/4000 x 20 which is .25 x 20 ...