Depends on whether the job was properly classified as exempt under the FLSA and you merely recently decided to voluntarily pay it overtime which then pretty much automatically makes it non-exempt by law.
If it was truly exempt (like a department head or VP) and you just opted to offer overtime, you owe nothing. If was truly always non-exempt per FLSA rules and you just messed up by improperly denying it OT and mistakenly paying it as an exempt, you owe the statuatory amount per the regulations.
Any situation in the middle between those two might have a different answer, like if was:
- an exempt foreman you want to offer straight time OT;
- or an exempt degreed professional technologist browned at not getting the regular overtime of her non-exempt non-degreed technicians co-workers sharing the same after-hours assignments.