Hi Colin,
If RE had given the Meteor a tachometer, instead of the occasionally used Tripper device, it would be much easier to monitor a proper break in period than trying to watch the speedometer for every gear change, in itself a rather unsafe practice, taking your eyes off the road far too much.
Once you reach the 300 mile mark and do the first oil/filter change, it is time to start opening that throttle up a bit more, varying the engine speed as you shift through the gears. Getting a "feel" for the sound of your engine as the revs climb is helpful when you don't have a tachometer.
The Owner's Manual says to keep the top speed (in 5th gear) to 50 mph from 300 up until the 1,200 mile mark. I had mine up to that briefly, on occasion, in top gear, before getting to 300 miles. No wide open throttle though. I believe RE's recommendation on the break in period is a bit too conservative, in both mileage and speeds. I will be using 1,000 miles as the full break in period.
Top speed of the Meteor is supposedly around 70 mph, so taking it up to 60 briefly (in 5th gear) after the first oil change should be fine. Just don't sustain it at that speed for any length of time. Varying the engine speed is most beneficial to break in, with the occasional burst of acceleration.
Rolling off the throttle completely for a few seconds, after a burst of speed, is good for the engine as well, as the sudden decrease of fuel mixture entering the combustion chamber creates a bit of a vacuum effect in the cylinder, which tends to draw oil up the cylinder walls for a bit of extra lubing while decelerating.
Hope that helps.