How does the plugin work out my side LED count?

Number of side LEDs (per side) lives on LED Devices > RPM Telemetry > Configuration and describes the LEDs either side of the centre RPM strip, so 3 for an X-10-X layout.

Every device keeps its own count, and the plugin works it out for you. A generic DNR profile draws side, centre, side across the whole strip, so only one number ever fits:

strip total = (2 x side) + centre

Your hardware fixes the total and the profile you installed fixes the centre, so the sides are whatever is left over, halved. Installing a DNR RPM profile reads that device's layout and sets the number for you.

That is why the slider is often greyed out. Where the count can be worked out there is nothing left to decide, and a slider you could drag would look like it worked and then quietly revert. The line underneath shows the sum and names the profile making the decision, for example "Cube Controls Phoenix has 16 LEDs and the X-10-X profile on it uses 10 in the centre, so 3 per side is the only fit".

To change your sides, install a profile with a different centre. On a 16 LED strip, X-10-X gives you 3 either side and X-8-X gives you 4.

It is fixed for a different reason on a wheel or DDU whose profile bakes in its own layout, such as a branded wheel profile. The page tells you, and every other setting still applies to that device.

The slider stays live only where nothing can be worked out, such as a device that reports no LED total. If you are entering it yourself, count one side, not both, and leave the centre out. A side split into more than one run is still one side, so a 9/3/10/3/9 wheel is 12 per side.

If a device has no side LEDs at all, the plugin says "No side LEDs configured" against the settings that depend on them. That is expected rather than a fault.

If it looks wrong on track, the symptom names the fault. The left side correct, the rev bar spilling into the right side and the right side effects sitting too far along, means the profile's centre is too wide for that strip, so install one with a smaller centre. Both sides wrong means the count itself is wrong.