It doesn't quite work like that.

You would need some methanol in there somewhere. If you have a fuel that is 25% nitro and 20% oil that would leave you with 55% methanol.
You also need to keep in mind that the nitro ratio that is important is between the nitro and methanol. This is what is burned for energy. The oil, no matter what kind, does not burn. It's purpose is to lubricate and to help cool the engine. By adding oil, you don't change the nitro to methanol ratio.

Also most folks would not run 25% nitro in one of these engines. 10% nitro would be plenty. At the same time, 50% oil would be far more than you would need. 22%-25% should be fine.

Robert