Keep in mind that it also depends on what you're trying to get out of your engine.

If you want peak power out of an ABC/ABN engine, then you want to get the piston/liner up to temp quickly and keep it there. Running the engine "4-stroking" or overly rich will wear the piston/liner fit a bit and reduce peak power. HOWEVER, if you intend to fly the engine at that kind of needle setting, then that's what you want! The piston/liner fit will work itself to be exactly right for whatever RPM and temp you run the engine at over time.

Break-in in the small 2-strokes is actually more about the connecting rod and it's bushings than anything else, btw. If you run lean during breakin, that's where things break. You need enough oil and cool temps on the conrod to keep the bushings from seizing and causing a broken rod, but you want heat on the piston/liner. So you use a small prop to avoid stressing things.

In an ideal world, for max power, you kind of know what kind of RPM you want out of the engine. Last one I broke in was a Magnum XLS .15 I'm using in combat, where I wanted max power. So I used a small prop and a tach, and ran that engine right up to the RPM I want later (20k) for just a few seconds. The needle wasn't fully leaned out, but it wasn't really rich either. I ran for about 10 seconds, then let it cool totally. Then did it again for several cycles with increasing run length. After a tank of fuel, I put on the prop I'm going to use in flight, and brough the engine up close to what I want RPM wise for a little bit. I then flew the plane a few times, taking it easy on the engine.

Here's a link to how Dubb Jett suggests you break in.
http://www.jettengineering.com/

But, as I said, that kind of break-in is what you want for peak RPM out of an engine. Most sport flyers aren't going to care if they "could have" had another 500-1500rpm out of the engine with different break in. And frankly, except for my combat competition engines, I don't either. The flip side is that if you run an ABC engine really rich for a while, then change props/fuel and try to really peak it out at high RPM, you'll get a lower RPM than someone who broke in for high RPM.

So, unless you're racing or something like that, just run the engine a bit rich. A tank on the ground, then to fly. With an ABC engine, that's all you really need.

Oh, you will notice that the idle will be a bit rough at first, and engines that use real chrome do break in slower (a good thing), and it sometimes takes a while before they idle well. Also expect the idle mixture and even the high-speed mixtures to change a bit as the engine breaks in. I have a TT .46 around here somewhere that really went through some wild low speed changes as it broke in. Of course, I got that engine used off of someone after they crashed it on the second flight. They thought they had a bad engine, what they really had was a rather nice, tight, un-broken-in engine.

And one last thing, most engines come with a piston that's been heat treated a bit, or maybe it's something with the alloy, I don't know. But there are a few, like my Webra .25s, where something odd happens after the first run. Before the first run, they don't seem to have a very tight "pinch" at the top of the stroke like they should. But after the first run, they get really REALLY tight. As I understand it, the high heat encounted by the piston can change the structure of a the metal a bit, and that can cause a very slight change in size and thus compression or "pinch". I mention this because normally you want a new engine to be really tight. When I got my first Webra, I was sure I had a lemmon, it was so loose. Then I ran that thing, and it became seriously tight and really put out the RPM. OS engines that I've used don't seem to do that, they always seem to be at their tightest when new.