Finished and packed for tomorrow. :)
The shoulder-strap became ridiculously long... I cut pieces from the fabric I had left and sew them together with a cotton strap and misjudged the length. :D Fortunately I can adjust the length and maybe I'll cut it to a better length some day, but not now. 
I have a meeting on Friday and I really wanted the bag finished until then, so today...no math! Or rather...no math yet.