The Ultimate Chest Binder is only two-layers and the armpits are the same shape (you can always modify these using an overlock machine, by the way). No one I know who is plus-sized has had an issue with the length of the Tri-Top creating a bulge - even in people much larger than you.
Some people have issues with either rolling up, but I am worried that the "lighter" one will not be enough to actually compress enough to make a big enough difference.
Bra size is a very ineffective way to judge what size you'll need, it's all about the actual body measurements. I think you know that, I'm just saying because telling people you're a 38DDD will unfortunately not help them help you. Because even if you've measured your bra size properly someone who says, "O, I'm that size and this worked for me !" might not have !
(underbust + largest bust)/2 is correct. It's always possible that the size the numbers say won't be right - that's why they offer returns.
I know someone who needed a size SMALLER after measuring that way. It will vary person to person, but the /2 is your best starting point and works for most people. Maybe a problem people have is that they round downward when picking a size - it's true that if you're in-between sizes or very close that you want to round UP.
It's just like with finding the right bra. You can do measurements all you want but at the end of the day you have to still try on a few bras to make sure the numbers were right. This is true of even the very best bra fitting systems because the shape of people's anatomy is so unique and the numbers alone can create a misleading picture.
Underworks has a good return policy or you could sell one that doesn't fit on here. I wouldn't worry about it ~too~ much. Most people get one that fits on the first try.