C-C from the woofer to the middle of the horn will be around 14", maybe 14.125 at the outside.
I would like to cross somewhere around the 850-900 hz mark, but not 100% sure until I get to build a mule.
2/3rds of a wavelength at 800 hz is 11.3" and my C-C will be around 14"... I won't be within the guidelines by D'appolito, but I figure close enough to give me decent performance. Distance is between the hf device and the midrange / woofer as per the article I have. ( Speaker Builder 1984
"You take the blue pill - the story ends, you wake up in your bed and believe whatever you want to believe. You take the red pill - you stay in Wonderland and I show you how deep the rabbit-hole goes."