Online Community of Zenith Builders and Flyers
It would be somewhere between 12 and 13 inches, close to 13 but not quite, actually.
To get the 1 inch flange you have to take into account the bend portion itself.
I have attached a spreadsheet that calculates the numbers for you (in mm not inches).
I have also attached Zenith's Construction Standards.
On page CS #301 (18 of 42) it gives a simplified explanation of what is going on.
There is an explanation in the spreadsheet also.
Hope this helps,