So you are using Luffy as the representation of Yonkou lvl?
That's tough to judge atm, based on his Egghead encounter with Kizaru, Luffy is atleast comfortably below an all out serious Kizaru imo. If I ignore matchups and scale the rest of the Admirals to Kizaru, this would put Luffy closer to the old legends than the Admirals, but whether Kizaru was an exception or not remains to be seen.
Luffy is just a notch below Admirals or he's in the mix with them but just loses to Kizaru, hard to say until we see his next big fight.
With Loki I think he can def cap at Old Ray and Garp, and it would make sense. Obviously at the very least these guys can give Luffy more than a good fight, so Loki would be fine in this range.