I have mine centered 5.075mm above the center point of the switch, putting it within the hole on the switch bottom that fits the SMD LED. 0606 LEDs fit. The opening on the Cherry RGB switches is pretty small, 2.65mm wide, and 2.25mm high. I have no idea how I arrived at 5.075mm, it's been 2 years since I looked at this stuff, but I just measured in Kicad, and that's what it says I set it at.
The PCB you've got in your picture uses a different method, where the RGB led is mounted on the underside of the PCB, shining through a rectangular hole cut through the PCB under each switch, and it shines up through the switch, so you don't really need to get it quite as exact as if you're going to put an SMD LED between the PCB surface and the switch. I'd just use 5mm up from the center stem, and make the opening however big it needs to be to match the LEDs you're picking. Those look like they're probably something like the SK6812 MINI 3535s which are 3.5 x 3.7mm
For reference, this is what I made with the little 0606 RGB LEDs and those measurements for placement...More
(https://i.imgur.com/fHkMpPW.jpg)