Some federal contractors use "beginning of year" data to establish the pool from which promotions are selected. This avoids having to add the employees back into the denominator. However, if you are only using a single "end of year" snapshot, the successful promotions should be deleted from the job group into which they were promoted, and added to the job groups from which they came. This avoids the artificial inflation problem you identified.
You can use this OFCCP audit checklist to ensure you're doing what is required to maintain OFCCP's regulations including VEVRAA, Section 503, and EO 11246. Or request a demo to streamline your compliance and recruiting efforts.