Simply put, what's the Normal Mode equivalent of the Heart of Fire Mode ideal AC of 72? If HoF monster BABs are capped at +52, how high can they get in Normal Mode? (Unique monsters aside.)

Never thought of it, actually, as it's rather easy to get AC of "way beyond enough" during Normal anyway.

I'd guesstimate it somewhere in the neighborhood of +32 or so (Xvim Knights, around level 20 fighter with +5 weapon, proficiency and huge STR score) so AC of 52+ should be plenty?

If you take a peek at what works for a HoF decoy, and just try to get as close as possible in regular mode, you should be just fine.