Hi James,
I get the problem and I didn't want to sound like authenticity-police, but sometimes there is reason to be historically correct.....the arming doublet being one of these cases.
The doublet is essential part of clothing for both the 15th as well as the 16th century, there are differences but more in slight detail/style.
The general purpose for doublets is to uphold the hoses. In the 15th ct it's still part of the undergarment, i.e. you wear a jacket over it.
Now an arming doublet as being the "undergarment for battle" has some slight differences compared to a normal (= civilian) doublet: the laces for fixing the armour. But nevertheless it's part of the common clothing, i.e. upholding the hoses which would otherwise drop to the ground. At least in the 15th ct where the hoses are rather ending at hipbone level; it takes another decades (starting around the 1490s) until the joined hoses reach OVER the hip which allows them to stay in place without connected with a doublet; take a look at Breughels peasant's paintings or some of the Holbein paintings for example.
A doublet is tricky, but once you get the design right, fairly easy to sew.
If you look at a modern jacket you'll see the arms meeting the body at the side of the shoulder, more on the rounding of the shoulder.
A doublet will not work this way, as it restricts your movements. You'll be pulling up the leg armour whneever lifting the arms.
Therefore the arms must join the body at the upside of the shoulder, I think it's called the AC joint, where the acromion and the clavicula meet.
connecting the arm with the body there gives enough freedom to rotate without pulling the whole garment upwards.
Another important point is the gothic waistline; it's at elbow level, slightly between the end of the ribcasge and the bellybutton. This is the point where the body bends sideways and there you have a distinctive point in gothic silhouette. Most of the medieval clothing off the peg doesn't have this distinctive waistline, you'll need it with the correct fit for your doublet.
A good description for 15th century clothing is the "Clothing Guide for Men" from the Company of St. George, download here:
http://www.companie-of-st-george.ch/cms/?q=en/Clothing_Guide_Download and then check pages 14ff. for some instructive infos on a doublet.
Another good (if not the best) DIY-Guide is Sarah Thursfields "Medieval Tailors Assistent" - feel free to ask for the pages about sewing the doublet via mail (
wuritter@aol.com) if you don't want to buy the whole book.