Acetone doesn't substitute for/replace nitro. They serve different functions. Nitro oxygenates the mixture, thereby increasing power (& heat), which is not a linear relationship to the percentile used. With the decrease in methanol percentile with the addition of significant percentages ie: >20% of nitro, you need also to move to a colder plug to maintain the flame due to the diminished catalytic effect from the reduced methanol.

Acetone's primary function is that of flamerate accelerant. I erred yesterday when I mentioned that it had no effect upon hygroscopic properties of methanol. It does when used in small percentages. Yóu will benefit more from using nitro than acetone if you must choose between the two on a cost basis. Almost everyone uses nitro in a minimum of 5%, but few use acetone. In NQ, whilst there's high humidity, the temps are also high such that the water won't come out of suspension. Experimentation reveals that methanol will combust OK with quite high absorbed water content.

For a home brew, stick with 5% nitro, 20% oil and 75% methanol. You can add 3% acetone if you want. Just be aware that if you use all synthetic, you'll have be especially careful to ensure that you don't tune lean because synthetic vaporises at a much lower temp than castor.