Whenever I teach a class, I ask the beginning bakers how they know how hot their oven is. They always reply, "I look at the temperature dial!" If you believe your dial, then you have the same degree of accuracy your Grandma had when she added another stick to her wood burning stove to increase the heat. Every oven is different and it is very rare if your oven dial matches the temperature inside. You really don't know how hot your oven is unless you have a thermometer hanging on your rack.
An oven thermometer is the greatest, cheapest invention that applies to baking. You can buy them anywhere you purchase kitchen gadgets. For a few dollars, you can hang that thermometer in your oven and open the door (or look through the glass) and see exactly how hot your oven is. Bakers often find it amusing when they get a thermometer and learn exactly how "off" their oven is. They then figure they will have to stop using the excuse "I burn everything" or
"my oven doesn't cook right." If you can do the math, just add or subtract the degrees you need to get you to the right baking temperature. What is the right temperature? If I can get my thermometer to register between 330-345, it is just right!