First, realize that when you set the controller to 300F, it is highly unlikely that the temperature is 300F. On some grills it will be higher than this; on some it will be lower. If I set my Ironwo0d 885 to 300F, it will end up around 270F, but yours is likely to differ.
When changing temperatures, I never change the setpoint more than 30-40F at a time. Thus, to go from 300F to 400F, I would move it up from 300 to 325, then to 350, then to 375, then to 400. My actual controller settings would go from 330F to 360F to 400F to 450F to reach a cook temp of 400F. Again, your specific grill might differ.
Always measure your cook temperature using a 3rd party thermometer and adjust the controller to achieve the desired cook temperature. The 3rd party thermometer can be an analog oven thermometer or grill thermometer, a wired temperature probe, or a wireless temperature probe. Just make sure the probe is close to the protein being cooked, yet touches neither the meat or any metal grates.
Please remember that cook times and temperatures are provided as guidelines only. They are useful for planning the cook, but every cook is different. Measure the internal temperature of your meats using a good instant read thermometer that will read with an accuracy of +/- 1 F and that will provide a response in less than 3 seconds. The "gold standard" in instant read thermometers is the Thermoworks Thermapen used by many restaurant chefs. However, there are many less expensive devices that provide similar performance. They just won't be as durable. Inkbird and ThermoPro are a couple of brands worth investigating. While devices with slower response times will work, the faster ones are better as you can check several parts of the meat without leaving the lid open too long.
