When I was eating bagels on a regular basis*, we kept them in the freezer. The spinner that rotates the food inside our microwave runs at 6 RPM so food placed on the edge nearest to the door returns every 10 seconds. To get the bagel thawed but not too hot to handle, nor too tough to eat, nor too frozen to separate the halves, I settled on 80% power for 20 seconds so I could twist it apart and toss it into the toaster.
I’m also a big fan of the plate reheat function on the microwaves I use at home and work. I bring frozen leftovers for lunch fairly often (small portions), and the reheat function takes it from semi-frozen to piping hot.
If I’m reheating in general I’ll use lower power settings to get more even heating. The on/off cycle allows the heat to dissipate from the hotter spots to the cooler ones. I also let food stand and stir it or rearrange it part way through reheating for the same reasons.
Gee, that’s also the standard reheating directions for most microwaveable meals.
*I’ve mostly cut out all sugar and refined white flour from my diet, and lost 40 pounds doing so. White bread is kinda flavourless now.