It's very hard to correlate the story numerically. One of the more difficult tasks to take on. But I'd say the fusion formula requires but a few things. These must be discussed at length.
1) You must figure out how strong you feel the Base Boys are and where they line up with 18. Does your fusion formula account for this? Great.
2) Super Saiyan Gotenks and his standing vs Super Saiyan 3 Goku. If the Base Boys and Super Saiyan Gotenks are acceptable, then you just divide Base Gotenks by Base Goten and now we are getting somewhere.
Who knows how fusion really works, but that's a decent start.