It's not a actually a system...I have a few formulas with different weight put on defense, offense etc, that gives a percentage likelihood of a home, away or draw outcome. What I do when I tweak it is how much weight to put on the different numbers. It's quite complicated to explain. But it's actually formulas I have modified from baseball and hockey and I have used them for years with a decent amount of success there...just need to find the best way to apply it to soccer.