Twenty would be a good percentage to work with. Big-blocks work better with low nitro anyway.
I found out that my K-45 was one of a few that were brought into the country illegally, that someone had brought them in without paying duty on them and it was a European version that wasn't made for high nitro. Not knowing much about these engines at the time and thinking that I was getting a great deal on one new in the box, I snapped it up. The engines are a work of art my looking at them but my main problem was blowing the rear bearing seal. When it went, it would take out the lower con rod needle bearings with it. When I pulled the head off I found the seal and a needle bearing wedged between the piston and the sleeve.
I called Al Hoobs, sent the engine to him. He did a complete rebuild for me and no labor charged!
I never got hte engine to run right, I hope thta you have better luck than I did. I've been told that the .90's are screamers and have been used with success in twin hydro versions.