No simple answer, particularly for those career AFSCs where performing parachutist duties is not inherently a primary duty for performing duties of the AFSC awarded. (I used career AFSC rather than duty AFSC to emphasize a few things explained below after the info from the DOD pay Manual.)
The actual reality forcing the jump pay to be prorated is connected to the reserve unit's access to availability of getting aircraft scheduled to jump from and if the scheduled jump doesn't get cancelled because of weather or aircraft scrubbing due to unexpected need for maintenance.
Table 24-4. Parachute Jumps-Incentive Pay for Parachute Duty-Performance Requirements, note 1:
Incentive pay for parachute duty may be paid, provided prescribed requirements are met, only from the date of reporting for duty or training. If that day is other than the first day of a month, that month's rate of pay will be prorated in accordance with paragraph 240201.
240202. Member of the Reserve Components
A. A member of the Reserve Components on extended active duty (EAD), who is ordered to perform any of the hazardous duties listed in subparagraph 240203, is entitled to pay based on the terms of this chapter.
B. A member of the Reserve Components on active duty training (ADT), who is ordered to perform any of the hazardous duties listed below, is entitled to pay based on paragraphs 570302 and 570401 and the terms of this chapter as affected by Table 24-1, rules 9 through 13.
DAFSCs that are jump-inherent do not require a J prefix.
3.4. Validating Permanent Parachute Positions Using Jump-Inherent Duty AFSCs. Certain Duty AFSCs (DAFSCs) require the performance of deliberate, regular, and recurring parachute duty to accomplish assigned missions. These DAFSCs are central to the ability of the USAF PPP to carry out its
assigned functions, and they provide a pool of qualified and current mission parachutists independent of the manpower positions in which they serve. In these cases, parachute duty is considered to be inherent to the DAFSC, and since positions are validated on the basis of recurring requirements, the intent of a J inherent position is that the member will maintain standards, qualification, and currency. Legitimate possession of a jump-inherent DAFSC constitutes a valid parachute position and exempts its holder from the
process described in paragraph 3.3.
3.4.1. Certain specialties require continuous parachute duty to accomplish the USAF PPP’s operational objective and to maintain award of the DAFSC. The following specialties are jump-inherent: 13DXX, 1C2XX, 1T2XX, and 1W0X1C (including AFSC transition to 1W0X2).
3.4.2. Permanent disqualification from parachute duty precludes service in a jump-inherent DAFSC.
Exceptions to policy are authorized by HQ USAF/A3O.
3.4.3. The exemption from position validation for jump-inherent DAFSCs does not extend beyond the DAFSC (i.e. secondary AFSC, tertiary AFSC, etc.).
3.4.4. While 1T0XX is not considered a J inherent DAFSC, members who hold the 1T0XX DAFSC require qualification to accomplish assigned USAF PPP objectives and are authorized to attend qualification training independent of the positions in which they currently serve. Members maintain active parachutist status, including currency, only when assigned to J coded positions or temporary parachute positions per paragraphs 3.3. and 3.5. The 1T0XX specialty is exempt from the requirements contained in paragraph 3.5.2.