When you PD an on-call day, your guarantee is debited by a "reserve pro rata share," which is defined as "the reserve guarantee divided by the associated number of on-call days in a bid period on a reserve line." So, regardless of the number of on-call days you personally start with, a PD costs you the same amount, i.e., 1/16, 1/17, or 1/18 of the reserve guarantee, depending upon the ALV and reserve staffing in your category.