Buyback for a subset of loans

Hello!

I'm trying to understand buyback. At this time I have two sets of loans, one set from my undergraduate degree which will be at 120 months in Feb 2027, and then graduate degree loans which will be at 120 months Feb 2029. My payment counts are up to date with the latest certification I sent in last month.

I would like to buy back 17 months of eligible employment during the SAVE forbearance when I hit 120 for my undergraduate loans in Feb of next year. Has anyone experienced buying back months for a subset of loans?

The big question: Do they base it on the % of your payment that would have gone specifically to your undergraduate loans at the time, and then figure out the formula from there for your lump sum?

This feels like the route that would make the most sense. If they based it on my full payment (that includes both sets of loans) then would I effectively be buying back the months even for my graduate loans not yet eligible for buyback?

I'm a ways off, so there is time to let all the SAVE stuff settle and we'll see if buyback is even achievable before I'd hit a clean 120 on both on my new payment plan... but if I could get even just my undergraduate loans forgiven with buyback next year, I'd be in a good spot with my graduate loans.

Looking for insight on this process from anyone who has been through it, or is currently going through it!

None of my loans are or were ever consolidated and I'm trying to keep it that way. I did the calculator for the process to consolidate and then have the count averaged, but did not like the outcome so that is not a route I want to take at this time. I also like keeping my loans unconsolidated because if there comes a time where I can no longer work toward PSLF, it is much easier to pay extra on smaller loans and snowball it than a larger lump sum.