I suspect that my - fairly new - oven gets too hot, so I bought an oven thermometer, which confirmed this. So I called the supplier. They say that in order to obtain the correct actual oven temperature, for any setting, you take the highest reading on the thermometer, and the lowest, and then divide both readings. Is this 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 thermometers 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 were 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.