Unless this model of Rossi is different than its brothers, 0% to 5% without a shim and a medium to cool plug will give you the best performance and reliability. If you have to shim it to run 10 or 15%, then it was obviously designed to run 0%. From my experience, the less the better in Rossi's, MVVS's, Webra's, Moki's. Even if you shim it to run more nitro, it will still run better on less without the shim.

I have a Rossi 50 that thrives on 0% nitro and 10% oil.