USGA handicap, simplified
Differential = (Score − Course Rating) × 113 / Slope. 113 is the average slope.
Best-N table: 5–6 rounds use best 1; 7–8 best 2; 9–10 best 3; 11–12 best 4; 13–14 best 5; 15–16 best 6; 17–18 best 7; 19+ best 8.
Multiply by 0.96 (the ‘bonus for excellence’) to get your index.