but I would bet the clearance should be roughly half the fan width
I was informed by a fan expert that it should be one fan blade width. That sort of made intuitive sense to me.
That is roughly 25mm. Actually a bit less if you think about it.
If you look at the top fan on the Noctua NH-L12 it is less, perhaps closer to 15mm. And if you run it with just the underfan, maybe it is 0 MM... or just in case 10mm - not for improved cooling - but to avoid noise generated by air turbulence through the longer path the air must otherwise travel through the fins. My guess is that the longer path might even improve cooling performance... bu at the cost of increased noise.