That 20% conversion only works in direction of loss from crank to wheels. For example we lose 20%. You cant reverse it mathematically to determine wheel to crank with the same factors.

Example: 20% taken from a 100 hp engine would read 80hp.

You cant reverse it by saying your 80 hp dyno reading can add 20% (16hp) . It would only equal 96hp.

I hope I explained that clearly..