Sorry, lacmag3, on checking this more closely, what I wrote above is wrong. It's not full difference, it's 90%. Here's what it should have said (I've also used your example handicaps):
You're playing a four ball better ball. Each player gets 90% of the difference between their handicap and the handicap of the lowest handicap player of the four. For example, if the lowest player is a 15 handicap, and the other three players are 20, 20 and 26 handicap, those three players get ((20-15)x0.9)=4.5=5, ((20-15)x0.9)=4.5=5 and ((26-15)x0.9)=9.9=10 shots. Therefore they get shots on holes with stroke index 1 to 5, 1 to 5 and 1 to 10 respectively. It used to be 3/4 the difference - i.e. 4, 4 and 8 shots - but in 2016 that changed to 90% of the difference.