"Incident meter directed straight at the sun: f/11½ (no surprise!)"
Let this be Zone V.
"Spot meter reading off wet blacktop: f/64½"
I get 5 stops here - so this would be Zone X. (Forget about all this 2.5 stops over 2.5 stops stuff.)
This should print as pure white and would qualify as a specular highlight, i.e. a highlight that has no detail.
Kirk - www.keyesphoto.com