I don't think I can help with your specifics (though I always use the middle rack unless directed otherwise) but thought I'd post this recipe, which I quite like and has gotten good reviews from others on the BB. I use a non-stick muffin pan and use a light spray of canola oil.
Garden Muffins
From "The City Gardener's Cookbook"
1 cup flour
1 cup yellow cornmeal
1 Tbsp sugar
2 tsp baking powder
1/2 tsp salt
1 egg
3/4 cup milk
1/4 cup oil
2 Tbsp butter, melted
1/2 cup peeled, seeded, and finely diced tomatoes
1/2 cup grated zucchini
1/2 cup grated carrot
1/4 cup chopped green onion
Preheat oven to 400F. In a medium bowl, combine the flour, cornmeal, sugar, baking powder, and salt and mix well. In a small bowl, beat together the egg, milk, oil, and butter. Stir in the dry ingredients. Add the tomatoes, zucchini, carrot, and green onions, blending thoroughly. Spoon the batter into greased muffin tins, filling about three-quarters full. Bake 20-25 minutes, until golden brown. Serve warm.
Makes 1 dozen muffins.
Happiness is not a goal, it is a byproduct. - Eleanor Roosevelt