5 C.F.R. § 550.1304
Overtime hourly rates of pay
(a) For a firefighter who is covered by (i.e., nonexempt from) the overtime provisions of the Fair Labor Standards Act (FLSA), the overtime hourly rate of pay equals 1
(b) For a firefighter who is exempt from the FLSA, the overtime hourly rate is computed as provided in § 550.113(e).
(c) For any firefighter, overtime pay for any pay period is derived by multiplying the applicable overtime hourly rate by all overtime hours within that period.