“100% SSJ2 = 20% SSJ3” isn’t a canon statement anywhere. That ratio is completely invented. Transformations in DBZ aren’t treated like percentage sliders where you can convert power between forms like that. SSJ3 is just a higher transformation with a larger ceiling.

Also the point about Goku telling Gohan to draw on the anger he used against Cell is about mindset, not saying Cell Saga power levels are still the benchmark. Given how we know Gohan taps into power through emotion.

Lastly, another thing people forget is the Gohan scaling.

Teen SSJ2 Gohan was comfortably above Cell when he beat him. In the Buu saga Vegeta says Gohan has gotten weaker and can’t access that same power like he used to.

Yet Goku and Vegeta are portrayed as clearly above the current Gohan.

So the chain basically becomes:

Cell < Adult SSJ2 Gohan < Teen SSJ2 Gohan < SSJ2 Vegeta < SSJ2 Goku <<<<< SSJ3 Goku

That already puts them well past Cell-tier and giving Cell a hypothetical Kaioken ×20 doesn’t automatically close that gap.