Super Boo and Ssj3 Gotenks can be assumed to be nearly even, so, roughly, Bootenks is 2x Super Boo.
Being said that, Chou Gohan overpowered Super Boo but was beaten up by Bootenks, so he falls under 2x Super Boo himself.
Given that, we have:
- Super Boo = 1
- Ssj3 Gotenks = 1
- Chou Gohan = 1.5-1.75
- Bootenks = 2
- Boohan = 2.5-2.75 (absorption work like addition of power and, at this level, Piccolo and the base kids are completely inconsequential).
In DB, a 1.25 gap is even enough to assure a pretty decent win in a fight and, from what is shown in the Boo saga, the kids (who are FPSSJ, remember) are not 2.5-2.75 times weaker than the adults, so Ssj3 Gogeta isn't, by proxy, on Boohan's level.
Again, all of this is confirmed by outright statements in the manga, from both Goku and Boohan thmeselves (and remember that Boohan has both Piccolo and the kids' minds, so he knows very well how Metamoran Fusion and the Ssj3 transformation work):
Chapter: 503 (DBZ 309), P9.2 Context: as Goku heads to Vegeta with the Potara Boo: “Now there’s another human with great power! But naturally he’s no match for me, even if they merged! ”
Chapter: 503 (DBZ 309), P8.8 Context: Goku tries to get Vegeta to use the Potara with him, but Vegeta refuses Goku: “Knowing you, I thought you might say that…! There ain’t any other way to beat Majin Boo!"
Also note that, unlike what happened against Boohan, Goku outrights tells us that Gogeta would be more than enough against plain Super Boo:
Chapter: 506 (DBZ 312), P13.1 Context: still talking about how they’re no match for Boo Goku: “…But there is one way we can win!” Vegeta: “You want to say Fusion, right? Well who cares about that?!” Goku: “Huh? You know about it?” Vegeta: “I saw it from the afterlife…You’ve got to be joking! You think I’d perform those ugly poses…?! Anyway, I thought I told you that I’m not going to merge with you a second time.”
Finally, you have to remember that, even after Elder Kai told Goku about Potara clear superiority over the Metamoran Dance, Vegetto was still amazed at his own power, which was greater than expected because Goku and Vegeta are also rivals, fact which enhances the outcome of Potara fusion:
Vegetto > Vegetto (expected) > Metamoran Dance
Given all these [I]facts[/], Boohan takes this.