I'd say i'm around a 22-23 pit in jackets as that allows me to have a jacket on underneath. I would probably go for a medium in SI as they tend to need you to size up.
Well i reckon you'd get away with it mate, not saying that as i want you to buy it but its that kind of jacket, it dosent give and im a 24.5 ish and its to small by a bit, i can hardly do the zip up