I fit a size 9 DB really well. After a few wears, I felt like I was swimming in a 403 in 9D (see sig). There was just too much volume all around. Ended up going with an 8.5 D in the 403 and, while snug at first, it fits me very similarly to the size 9 DB. Go figure. Further evidence that trying on the specific shoe in various of sizes/widths is the only real way to go if you want the best fit. The problem I encountered was, and the CXL 403 is my only Alden experience, I wasn't able to tell whether the fit was proper after only a couple short wears walking around on a 5'x8' rug. By the time I realized they were too big, I had already worn them quite a bit. I'm sure this is due in part to my own inexperience with this brand/leather. And I understand the recommendation to not buy a shoe that is uncomfortable in any way when brand new. But for me, there was a pressure point on the outside of my foot when I first started wearing the 8.5 D 403, but that pressure is no longer present after 2 weeks of solid wear. The boots fit great. If I had gone to a 8.5 E, I'm pretty sure it would have been too wide in the heel. That lead me to the conclusion that a) 8.5D is the correct size in a CXL Indy boot or b) the TruBalance is not the best last for me. So, basically, you have to take into consideration not only the last, but also the material, as well as the type of footwear (shoe, chukka, boot).
If I had to do it over, I think I would have been better off shopping for a used 403 on B/S (seems like you don't have to wait long until the size(s) you're looking for will turn up. Particularly with this leather, a little beausage is not going to detract from the quality or value of the boot. This way, you can be a bit more aggressive in determining your size than with what the Mac break-in method allows for. Plus, you're saving money up front, and again if and when you resell them due to them not fitting.
Haven't been around here long, but this is sound advice. Trust me. (And I'm not just saying that because I'm selling my gently used 403's )