A simpler formula for calculating your royalty acres is:
1/8 - 1.0 royalty acres per NMA
1/6 - 1.33 royalty acres per NMA
3/16 - 1.5 royalty acres per NMA
1/5 - 1.6 royalty acres per NMA
9/40 (22.5%) - 1.8 royalty acres per NMA
1/4 - 2.0 royalty acres per NMA