Please keep in mind, I'm recommending the lower nitro at 20% castor ONLY for the break-in period. The lower Nitro will produce less heat at an identical needle setting, yet hot enough to produce desired wear for break-in. You will not be running so much fuel to break-in the engine that a 5% jump in nitro would be ineffective. My main thoughts when breaking in any engine is to make sure it's well lubricated, has just enough nitro to run well, and don't over-prop it.
I know that others have their own thoughts on this, but I have found this to be effective for many, many engines.

ED