The simplest and easiest resolution that is most fair is straight years of service.
No windfall for one group.
If you have 15 years of service, you go ahead of someone with 10 years of service.
Retiree with 30 years of service goes ahead of active employee with 20 years of service.
Retiree with 25 years of service goes BEHIND an active employee with 30 years of service.
The more time you give to the company, the higher your priority. Period. Who would disagree?