Unless you have an inverter microwave (most don't) there's no such thing as 50% power. It's 100% power for 50% of the time. The first 10 seconds might be power on, the second 10 you might just be watching it spin around for nothing.

This is correct -- it's usually a cycle of several seconds on, several seconds off, so 50% power for 10 seconds is somewhat nonsensical. Just do 100% power for something like 7 seconds at a time, maybe, if you want to be really careful.