Made initially for Dungeons & Dragons, but the principles transfer well to other games. A method of calculating an NPC’s HP, AC, to-hit, and average damage so as to provide the PCs with an appropriately challenging encounter more consistently than the in-built CR system.
Let n = 0 for mooks, 1 for tough mooks, 2 for leaders, etc.
Enemy AC should be the average To Hit bonus of the PCs + 10 + n.
Enemy HP should be 2 + n times the average damage roll result of the PCs.
Enemy attack bonus should be the average PC’s AC + n - 13.
Enemy damage should be 1+n/4+n of the average PC’s health damage on average.