Oven Temperature: How to be sure the selected temperature is correct.
The "correct" temperature is determined by using a properly calibrated thermometer. This will likely not match the 'setting' which you have dialed up on your stove because oven thermostats are notoriously unreliable. You should be able to arrive a correlation between the setting you use and watching the thermometer you bought. Remember that each time you open the oven a significant amount of heat is lost.

If you can place the thermometer where you can read it through a window into your oven and make note of the setting/actual you should be able to arrive at a useful guide to your oven. 

That said, there is often very little difference between cooking something at 325F vs. 335F. But I would recommend you depend on time@temp less than internal temperature (using an instant read thermometer) to correctly gauge the 'doneness' of your dish.