Not to throw a monkey wrench into these works, but there’s something I think you aren’t necessarily considering: back length is really hard to judge because not all jacket collars sit at the same place on your neck. As an example, a size 40 of BK’s relic hunter has a back length of 23”, and actually sits lower than a Dubow (24.5”). The Rough Wear 23380 in the same size has a back length of 25”, but actually feels shorter than the Dubow.
I wouldn’t assume that you can know what back length you need in a given contract until you try it on. Andy, meanwhile knows these jackets backward and forward and had fitted a LOT of people. My advice would be to give him your dimensions (height, weight), with some pictures, and maybe the measurement from the base of your neck to where your back meets your hips (the belt line on most modern jackets), and let him guide you.
Interesting point, thanks. Andy advised adding no more than 1 inch to the Dubow, and he has seen the photos of me in the 27 inch jacket.