FAIR LABOR STANDARDS.
29 CFR § 541.604
https://www.gpo.gov/fdsys/pkg/CFR-2009-title29-vol3/xml/CFR-2009-title29-vol3-sec541-604.xml
See text below. They are touching dangerous ground with a bonus tied to hours worked and divorced from actual labor rates. Lot of problems with their approach.
There is more to this.. it is complicated.
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Minimum guarantee plus extras.” That section provides that so long as an exempt employee receives a guaranteed salary of at least the minimum weekly salary level (currently $455 per week), providing an extra payment above the minimum guarantee does not jeopardize the employee’s exempt status. There is a caveat however. Under 29 CFR § 541.604(b), if the extra pay is computed on an hourly, daily, or shift basis, there must be a “reasonable relationship” between the guaranteed weekly salary and the amount that the employee actually earns. The regulations state that a “reasonable relationship” exists when “the weekly guarantee is roughly equivalent to the employee’s usual earnings at the assigned hourly, daily, or shift rate for the employee’s normal scheduled workweek.”