This is what I do. I install the extra head shim and use 10% Nitro, 20% oil (50/50) castor/syn, balance methanol.
I use a 9x6 as the Rossi's like to turn the rpms. Crank it up at full throttle and screw the HS needle in to a clean 2 cycle (20 seconds) to warm it up a bit and straighten out the
liner. Then back it down to until it breaks into 4 cycle then back up to the point where its bouncing between a 2 and 4 cycle. I run it for an additional 2 minutes then shut it down to cool off. When its cold open the needle a half turn, then crank it up again for 3 minutes readjust the needle to the point between 2 and 4 cycle. I run it for a total of about 12 minutes (4 runs) then start pinching the fuel line to bring up the rpms a bit at random. After another 2 runs I will then set the idle screw as Rossi's always come filthy rich on the low speed needle. Once its running real good and has a great transition I will then fly the engine on a plane. After about 30 more minutes you will notice that the transition is a bit sluggish again. Its just the engine becomming fully broken in. Reset the low end and it should be good for a very long time.
I also take the engine apart prior to the first start and check it inside for metal debris ect. Gives you a chance to lube it up a bit as well
Rossi's are really great engines and once you get them broken in and tuned you will have a long lasting high output engine. Some of mine are 8 - 9 years old and still running strong
