Trying to mix castor to fuel for 1/2a cox engines

is there a math formula which calculates the % of oil in fuel to add to a batch to end up with N% oil in fuel and what the added castor decreases the nitro content to?

Example.

32oz car fuel 35% nitro 12%oil

If I added 3ozs castor to the 32 oz fuel I end up w/ 35ozs total mix
it would bump my oil up to 19% but dilute my nitro to 32%- (this is in strict adherence of the laws of mathamatics in accordance to RocketRob)

I sortof fumbled through this one and wished I had a real formula to solve for both the amount of oil to add to end up with a predetermined oil percentage and the effect on nitro percentage.

Is nitro available as an additive to bump the nitro back up?

Thank you in advance - my brain is tired