There's not a correct amount. It's based on your own feet. Like bkotsko linked, it varies due to the different measurements. My heel-to-toe Brannock size is about 12.6, my heel-to-instep is 13, and my width is C. I have 12D Barrie shoes and boots that fit fantastically and have about a thumb width of space. I have 13D AEs in their 5-last, and the amount of space is closer to a thumb width and a half, but it's the right size for my instep.
I too am 12D Barrie boot and AE 5 last I am 12EEE for bal's and 12E for blucher..... so it is hard to compare just on the numbers