The percentage of nitro doesn't matter when breaking in the motor. Use whatever you're going to use the rest of the time, or use whatever you have on-hand, just be aware you'll have to re-shim and re-tune the engine if you change to a different nitro percentage. I use 30% nitro in all my vehicles, because it saves me the trouble of re-tuning when the weather changes a few degrees (which it does every single day in the Mid-Atlantic), and I just add a shim or two to the engine before I even break it in, because I know it'll need it.
What matters during break-in (and the rest of the time you own the engine) is oil content; never run with any fuel with less than 10% oil, unless you're racing and you re-tune your engine to suit ambient conditions before each race day. The extra oil will provide extra protection against damage caused by brief overheats.
For the first run, tune it so rich it won't rev-up enough to go faster than about 10mph. After that, start tuning for a peak temperature of about 200F, then 210F, then 220F, 230F, 240F, 250F. When you get to 250F, keep tuning to maintain that as a peak temperature; when the engine finishes breaking in and "drops its nuts", you'll notice it will start to run cooler and you can tune it leaner without exceeding 250F. Likewise, if you run in grass, you'll have to tune a bit richer to keep from overheating due to the extra drag from running in grass, and this might cause your on-road temperature to never exceed 230F, but that's fine -- it's overheating that damages the engine, not underheating, at least not by small amounts.
Remember that nitro will never produce as much torque as electric, and accept however much power you get at 250F, and don't tune leaner to get more power. It's WAY too easy when running off-road to end up at 300F+ from running slightly lean and having too much fun. (I once ended up at 413F because I tuned for peak power without leaning-out on acceleration, and then I drove off-road for 5 minutes. The engine never worked right again.) Once I understood this concept, my engines stopped losing compression after a couple months of regular use.