Figure out your actual DPI output, as it's different depending on what mouse and mousepad you use, and get the ratio of difference between your actual DPI and the DPI setting you set your mouse so you can convert other DPI settings as well ( the ratio is usually consistent after changing DPI values).
Basically to find your real DPI measure your cm/360 with a ruler and compare it to the output you get on the calculator after inputting your settings, then adjust the dpi setting until the cm/360 output close matches the measured cm/360.
it's as accurate as close as your measured DPI is to the DPI setting on the mouse.
On some mousepads I'd get 15% drops in DPI compared to the setting it's supposed to be on with certain mice/sensors, certain mice with the same sensor and supposedly similar firmware from the same manufacturer had 8% dpi differences, on odd razer mice on certain pads would be 2-4% higher than their quoted setting.
An example would be the WMO having a 430-450dpi output for some people, when I tested it with a good few pads, I got 400 on some mousepads, and 420ish on others.