I have a 689 as well, great jacket. Sizing is tricky as you point out as Schott recommends you go up one size in this model. I ordered one size up and wish I had just gone with my regular suit size. I kept mine and use it to layer in the cool weather.
I bought mine about 14 years ago. I took their advice also and bought a 42 instead of a 40.
The 42 was a bit large on me. I also lost some weight over the last two years so it was just too big and had to be loved by someone else.
I did all the hard work breaking it in for the guy who bought it.
I do kind of want to buy one again in a size that fits but I am afraid I may not like it. I fought with the one I had and wish it had a higher arm hole placement but blamed the large size, not the jacket.