Just to set the stage: I get traditional, full-cut 40Ls. Slim-fit anything has just never worked on me. 40L Hickey Freeman Beacon is just about spot on, maybe with a teeny tiny bit of extra wiggle room to go up (like a 41L, or a 40.7 if that makes any sense). 40L Hugo Boss is just a tad too small in the waist, and from what I gather, that's supposed to be a slim-fit youth-oriented brand. 40L Ralph Lauren Blue Label is also too slim. I weigh less than either of these chaps in this thread.
Using this guide (which actually appears to be the oft-recommended Ask Andy About Clothes guide) and measuring well-fitting jackets of my own, I find that my needs are a chest measurement no smaller than 22" (ideally a couple fractions or so more) and a waist measurement of around 20 1/2", again, maybe a notch more. The aforementioned 40L HF Beacon fits this category good.
My interest in BBs stems from this listing from tweedydon. Nice, friendly, communicative guy. I asked for additional measurements and questions of concern because the measurements just weren't saying 42L to me (not at all to question tweedydon's expertise since he's obviously experienced--it's more an analysis of the strangeness between tagged sizes of different jackets). Any 42L I've ever gotten hold of was always a tent on me, obviously too baggy, yet the measurements there are quite too small. 21 3/4" chest and 19 3/4" waist put it well into the too-small category, yet this jacket's Madion cut is supposed to be BB's fullest cut. Surely I don't need a 43L BBs and a 40L HF (and most anything else)? Yet that's what I gather here. Getting a slimmer fit would only mean I'd even need to go higher (44L BBs--surely preposterous). tweedy says it's unaltered, so slimmed-by-alterations is out the window.
The thread I linked to before seems to rather strongly suggest that, given my height and weight, I'd actually be be getting a jacket too large, not one too small. Nevertheless, after talking with tweedy, he advised that I pass. I don't want to disrupt that goodwill by buying it (for now...) anyway only to possibly regret it.
Everything taken together in context: If I wanted to buy a Brooks Brothers sport coat online besides tweedy's, what size and fit would I likely need (given that 40L HF Beacon is an established base)? And second, is there any chance in the slightest that tweedy's jacket would fit on me (I'm still eying it from afar)? I could simply email BB and see what they think, but I want to talk to people that own and wear the things, and can perhaps compare them to other jackets (like HF), something BB can't do. Besides, they'd likely want my measurements to determine fit. I could only give them measurements from competing jackets, not reliably measure myself.
Perhaps if someone had a 42L HF and 42L BBs Madison they could compare side-by-side, that would also help.
Thanks in advance for your time and consideration.