Light wavelengths. I know what I'm seeing, but I need the equation which explains it.

Biologist here whose exposure to physics is only in high energy x-rays, so dealing with the visual stuff is confounding.

I have two light emitting substances. Both excite at the same wavelength so that variable is controlled. One emits at ~525nm (+/- 3nm) and the other at ~620nm (+/- 3nm). Individually both excite and emit right where they should.

When they're stacked or intermixed the combined emission range is ~575nm (+/- 5nm.) So what began as green and orange emit yellow. I've replicated this a half dozen times and it's clearly not an artefact...the two wavelengths appear to be meeting somewhere in the middle so additive colour mixing is clearly doing its thing.

My issue is finding the equation which describes what's happening with the wavelengths rather than the colours themselves...everything I'm finding is the R+G=Y level of explanation rather than equation which actually explains what's occurring numerically. This is just a minor point in a much larger paper but I'd prefer that it not hit peer review with a kindergarten level explanation. What equation do I want to use for additive colour/wavelength mixing?