The surcoats are quite simple, so I just sort of winged it in making them. Although I did take the liberty of sewing the sides together at with wait for a few inches. It's not the way surcoats were made, but it looks much better and the sides don't move to give a sloppy look.
I'm assuming the part I bolded above was ment to say "at the waist"? If not, then I'm not sure what you had intended to type.
Actually the surcoats worn over mail were joined at the side, or only partially slit from the armpit down to close with laces. From the bottom, the only slits were front and rear, primarily to facilitate sitting on a horse.
The tabbards worn with later period armor, after plate had become more standard, could sometimes be open on the sides and held together with a belt.
A jupon worn during the transitional period of the 14th century would be very close fitting and not much longer than waist length (sometimes just covering the hips, sometimes not), and these would have a lace closure somewhere, probably on the sides.
Long story short, the long surcoats should be mostly closed at the side and have a narrow arm-hole if possible. So I think you're doing them right. Ideally they should flare out wider at the base than at the waist, which can either be done with the shape of the front/rare panels, or with triangular gores inset at the bottom.
