Your mouse pointer MUST stay within the confines of the monitor(s).
You do not have to start dragging the crosshair at the exact center, just click anywhere where the mouse pointer turns into a four-way arrow.
The browser zoom-level must be 100%.
Real Mouse DPI Analyzer
What is your real DPI?
With this tool you can measure the real DPI of your mouse, and calculate the sensor accuracy. It can also be used to find out what your DPI actually is if you don't have a setting for it in your mouse driver. UPDATE!
You can now calculate with different WPS (Windows Pointer Speed) settings. This means that you can measure over much greater distances! Make sure you select the same WPS in the calculator as you have set in Windows.
A blue target crosshair will now visually show you where 100% accuracy is based on your input. If if goes off-screen and the message Out-of-range appears, adjust your input to get it back on screen!
The measurement area will now scale to the edges of your browser, even if you resize it across multiple monitors.
You need to accurately measure how far you move your mouse, so do something like drawing inch/cm lines on a piece of paper and use it as a mousepad.
Enter the target distance for how far you are moving your mouse.
Enter the current DPI setting, if known. You do not need to enter this, but it is necessary for the deviation calculation and the target crosshair.
Click the red crosshair, and hold the mousebutton down.
Move your mouse the target distance.
Release the mousebutton to stop recording the movement.