It's really a term used for a multiplication higher than 2 but still within single digits. I'd think it's under 8 or 9 times seeing as 8-9x would likely just be rounded up to "nearly 10 times"
Goku is the one who says his power increases multiple times when using Kaio-ken:
http://comic.dragonballcn.com/list/0.Dragon_Ball-buyao_daolian_ya/0.jp_Kanzenban/DRAGONBALLvol16/035-dbvmP.jpg
Boo said his plan began while he was fighting Gotenks and he can't have sensed shit inside the Rosat so he must have felt Gohan's power growing just as the ritual was about to be completed
I'm ok with a 10x multiplier after the Freeza arc. Only problem is that it'd make SSJ weaker than KKX20, and Goku consistently treated SSJ1 as his max power (outside of the buff transformations) until he unlocked SSJ2 and SSJ3.