lol! You should get an oven thermometer, just to check. (They're cheap.)
I'd say that, with really large potatoes, you may need to bake them longer. I wouldn't increase the temperature any higher than 350. You don't have to wrap them in foil. It just makes a softer skin if you do. Always poke some holes in them before baking. The best way I've found to cook one in the microwave is to stab it all over with a fork, place on a plate and microwave on high for 5 minutes. Take out and wrap in a square of aluminum foil and let sit for 5 minutes. Then unwrap, add butter, salt and pepper, etc. and eat.