The NFL starts by calculating a "franchise tag" amount for each of the prior five years. The franchise tag amount is the average of the top five salaries at each position for each year. Then the NFL adds up the last five franchise tag amounts and divides them by the prior five total salary-cap amounts.
The NFL takes this rolling five-year average of how much of the salary cap that position's franchise tag takes up, then takes that percentage of this year's salary cap as the non-exclusive offer amount.