Goku factoring the RoSaT is nonsensical, considering that upon Piccolo bringing it up Goku dismissed the idea saying they can only use it once in their whole lives and that they better reserve it to another situation, implying he didn't think RoSaT was needed...
... but still, what withheld stated about "perfected the Gotenks' multiplier post RoSaT" isn't as nonsensical as one would believe... first off, we know that Goten and Trunks in the Room didn't power up nearly as much as Gotenks did, considering upon seeing Goten and Trunks laying out in their base form Piccolo did say nothing and was still hoping for a miracle... yet when he saw base Gotenks formed he was already goshing over this latter's improvement and believing they would have a chance, which kind of shows Gotenks' improvement was way bigger than Trunks and Goten's separetely, as this "big ass" power up was just made obvious when they turned into Gotenks. That would then imply that the boost the kids receive when fusing into Gotenks is simply considerably bigger than they one they did prior to the Room, as now we have a "slight" difference between the boys pre and post Room and yet a pretty big gap between pre and post Gotenks, which would just be possible if Gotenks developed on its own...
Also, like many people would believe here, the adults aren't way stronger than the kids here while in same forms (like, not "many folds stronger") considering Goten could already have his brother on his toes while in their sparring session... and yet Goku was sure that a fusion between himself and Vegeta would be strong enough to deal with Super Boo (to whom the kids post RoSaT were on par with at their best), plus the fact that Goku himself seemed also pretty sure that a fusion between himself and Gohan (which should likely be as strong as Gogeta considering Gohan would have to suppress himself to Goku's level) would work out against Bootenks, which would then imply that a fusion between Goku with somene with the same power as himself would benefit from the same boost as Gotenks did post RoSaT, which would then lend credence to the theory that Goku was basing his estimations on a Gotenks benefiting from the same boost as an "adult" fusion would, hence his estimations matching that of a POST RoSaT Gotenks rather than the pre RoSaT one...
...although yeah, upon sensing SSJ Gotenks' chi, Piccolo didn't mock his ability to take out fat Boo, rather he stated his chi was incredible and just wished to test out his speed, which then seems to compliment Goku's estimations... *shrugs*