Thanks, guys. I have a charcoal 24 Oz. Mackinaw on order. I went down to a 40 for a trim fit. I have found if I order a 42 vest (or large) , there is more room than I need, and plenty of room for cold air to find its way in. Also, a size down fits better under the MC jacket.
A snug fit for a vest inside a MC jacket is always best and think that you will be happy with the 24 Oz. Mackinaw. Farm-Ways in Vermont also has a close out sale on their moleskin vests and has one size 40 left. http://www.vermontgear.com/820F.html