Can a microwave oven have its output controlled, limited, by something
like adjusting the AGC on a TV? Or on combo devices, radio,
cassette, CD, tv, there is usually a pot on each of those, adjusted so
the volume stays the same when one changes functions.

I didn't like my old small microwave oven because it was so small, and
I also had to cook longer than instructions said, because it was low
power. (So I was in the habit of mulitplying the time given by 1.1 or
1.3, but I was hoping not to have to do that anymore.)

Now I have a new, one-year old, full-size Sharp microwave -- I can
provide the model number if it matters -- and it is too powerful.
Things cooked in their packages according to instructions have their
water boiling over and taking some of the food with it, and I presume
other food is being cooked more than instructions or recipes call for.

It has a power control that will lower the power by 10% for every push
of the button, but I would have to use that button every time.

Is there any chance I can turn the full power down by 10%, for every
use?

(As an aside, it's interesting that on 90% power, for example, I can
tell when the microwave part is functioning because it makes more
noise during the 90% than the 10% of the time. The old no-name-brand
only had one other power level, Defrost, but it made the same noise
all the time, and to see how much of the time it was making heat, I
had to turn on a nearby AM radio and tune to a weak or no station, and
listen to the radiation from the microwave.)