Hi,
i didn't have much time testing it but it should be very easy to set the Cluster finder up for the new Holy Radiance:
Range: 10 yards
Operation mode: Radial
Source: target of spell
Destination: member of raid (not group like it would be for PoH)
Now you let the Cluster finder Icon show up on your frames (Frames --> HoT-Icons take a slot you want and set it to cluster finder)
Edit: Made a quick test with a mate on PTR- and it works, there is an icon for every member in the raid and the number tells you how many people it would hit when you cast it on this exact target (so you want to cast it on the target with the highest number ofc)