How to know what watts to set for Microwave for 350F?

All microwaves cook much hotter than 350, no matter what the setting is. The power just determines the intensity of the beam (how quickly and deep the beem will heat). If you are cooking something big (like a pot roast or a whole chicken), i wouldn't try microwaving, as you will never set it right so that it cooks evenly. If you are trying to bake, its totally hopeless for the same reason. You'll have better luck with a toaster oven. As a rule of thumb, I'd try lower first as the microwave beam is less intense and wont fry the edges as much. Start with defrost and dont go over 500.