@dert
This was tough for me as well. I didn't have any Ship John pieces when my Wills order was ready for sewing. I measured a bunch of the jackets in my closet and ended up ordering the Large size Regular length Wills, mostly because I have a leather jacket that has similar measurements to the Large Regular Wills and I thought the weight of the leather jacket would be similar to the weight of the waxed canvas.
I am 6'-1" and 190 lbs. My UHFs are size Large. I can easily wear a Large UHF under my Large Wills.
I'm happy with my size choice. Best advice I can give is to be guided by the measurements of the other jackets you have and that you like the fit of. Hope this helps. I think they have a remake policy if you order something and then change your mind to go bigger or smaller, so, worst case, you'll get the size you need a couple of months later than your first shot at it.