Well, let's run some numbers, and see what we can learn:
If you're a level 1 Variant Human Fighter with 16 Strength, Great Weapon style, GWM, and a greatsword, your value for X is 11.33 (4.16+4.16+3), and your base to-hit bonus is +5. You gain DPR using GWM in cases where you need to roll a 10...