If you're getting very serious about your 'claying', it's worth thinking about getting a dedicated oven.
They're really quite cheap (compared to some of the other kit) - mine cost around £50 - and it means you can set it up exactly how you want it. You also won't have to worry about mixing food and polymer clay in your kitchen oven.
This would probably include an oven thermometer, and a few ceramic tiles placed on the base to try to even out any surges and spikes in temperature.
If you're getting one, try to get a decent one with a 'fan' oven - they're not really fan ovens (the elements are at the top and bottom rather than in the fan itself) but the fan helps move air through and probably prevents markedly different temperatures in the top and bottom of the tiny oven.