Limit effect with luminance mask

In Fusion, I'd like to limit the effect of the OFX Aperture Diffraction node (or the Highlight effect) on the highest highlights in order to prevent them from blowing out... I want the effect to be effective between, say, 80% and 90% of light spots.
I tried with the luma keyer but didn't succeed. The only way I found is to use the Ranges Mask, but it's really difficult to manipulate and master.
Maybe I don't link the different nodes properly...
Any idea?