Have you tried contacting them to see if you can get a custom sleeve length? Their advert implies that these are made to order in Scotland, with a lead time of 4-6 weeks. So it's possible that the answer is as simple as telling them what sleeve length you want, rather than suffering from "standard" sizing schemes. Best thing to do is measure precisely on a jacket that you already have to determine the sleeve length that works perfectly with your 23" shirt sleeve length (as measured from the shoulder seam) and see if they can just do that. There's no sense in ordering a brand-new jacket and then having to alter it when it comes in. That's just crazy talk! Make 'em do it the right length for you.

On the fit of the jacket itself, the customary "order one size up" advice needs to be taken with a grain of salt. If you haven't seen it already, check out this recent discussion on the topic. There's a link in that thread to another web page where more info can be had on chest sizing. Armed with that information, take another measurement of your chest, and perhaps some of the jackets you already own, and determine exactly what kind of fit you want. Then ask the STM to verify for you what the actual pit-to-pit measurement is for their jacket measurement scheme. You may end up wanting to alter your order once you have all that information.

I hate to say it, but it seems that most jackets I see people wearing simply don't fit them. And I think this is probably the result of confusion on how to order one, or vast inconsistencies on how they make them. Chest sizes are too large and sleeve lengths tend to be too long. Don't set yourself up to spend that much money on a product that isn't going to fit you well, or that you'll have to alter in order to wear. The fine folks at the STM will probably work with you to make sure you get the level of customer satisfaction and help that you need to be confident that your jacket will fit you exactly as you expect.