Mechanically speaking, most oven thermostats are simple devices. Recalibrating one depends on just a few things: an accurate temp reader (IR gun or digital thermometer with wire sensor) and a screwdriver.
Process goes something like this:
1. take a benchmark read: set it as high as it will go, and read the actual temp in the oven. You may be surprised at the amount of variance in your oven. Most thermostats cycle in a 50-degree range: ie, if its set to 500, it may go up to 525, cut off, then come back on when it drops to 475. Its all about averages.
2. tweak thermostat. There are either 2 screws behind the dial, or a small screw inside the D-post if you have a D knob. Those screws regulate how far the triggering element travels. If you've ever taken apart a house thermostat and seen the mercury level, its a little like that. What you want to do is increase its range upward by a certain increment, say 1/6th or 1/7th of the dial. Then do the temp test again. Take your time and be patient with it. Repeat as needed to get to where you want.
You may get substantially different temp vs time results after installing the firebrick insulation. After doing that, your oven will take longer to heat up, and longer to cool down, so the thermostat will cycle less once it gets to temp. But the basic temp control will be unchanged.
