You could fit something like this:
Gotenks: 1
Boo: 1
Gohan: 1.5
Gotenks-Boo: 2
SS3 Gogeta/Gokan: 2.25
Gohan-Boo: 2.5
But that is too tight for me. I just prefer to believe that Gokan would be stronger. There's also this quote:
Chapter: 469 (DBZ 275), P6.1-7
Context: after Goku laments that Gohan and Vegeta are dead
Goku: “So you know about it, Dende…! That’s right, some people from Metamor who I met in the afterlife taught me that art…It’s a merging technique which can only be performed if two people are fairly close in both power and body size…In other words, by having two people merge into one, they’re able to become a single, new human with amazing power which either of them on their own absolutely wouldn’t be capable of. It really is incredible! Those two from Metamor were completely weak and gentle on their own, but by using Fusion they transformed into a substantial warrior! [ ] …I was just taught the art, but I ain’t never tested it out…There wasn’t anybody on par with me in the afterlife…”
Based on the underlined, I would say Gokan should be way stronger than Ultimate Gohan, while I don't think Gogeta would dwarf Gohan in the same manner.