This is, thankfully, pretty easy to answer.
My understanding is that .5 RPP and 2 RPP are settings conjured up by default by switching from high to epic. If you want the specific lumen scalability CVars, Epic already has a page setup for lumen performance optimization:
That being said, the concept of ‘RPP’ is a little meaningless with lumen GI. They aren’t actually tracing a ray per a given pixel for diffuse lighting; instead, it’s referring to the # of rays shot per screen probe, which is then interpolated to the Gbuffer for diffuse lighting. Just a bit of technical minutiae, but still.