I have been saying this for years: If you care about authentication at all, read your sources BEFORE you start the project. Then you will never have to stress about backwards-justifying your work.
Addendum: Even if you don't care about authentication at all, STILL read your sources BEFORE you start!
I have been holding onto the remains of a red silk shirt for years. It's silk! Must! Use! Silk! For Something! I finally decided that the center back panel could be cut down to make a Viking coif/cap/hoodlet, a la the Dublin or Coppergate caps. This isn't for a competition or anything, and the cap is dead simple - a rectangle folded over, with a clip at the back-top corner optional. So I can just do it, right?
I had my lovely new cap, the edges all hand-rolled and hemmed, the top a French seam, and a lovely six-strand braid of blue silk decorating the bottom edge. I put it on, and...
It was long. Way, way long. Even longer than the folks who make theirs extra-long so it keeps the sun off their necks. Like, so long. Also somewhat deeper than it needed to be.
I bumbled my way back to one of the web pages I knew I'd seen before - Jennifer Thies' article on her Coppergate coif. And lo, there were dimensions given. And LO, when I pinned my red silk thing to approximately those dimensions, it skimmed the tops of my shoulders just like it's supposed to do.
It's not really that big a deal - it's no more than a couple hours' work to re-sew what I've done, and I can do it without having to remove and replace the decoration. It's just forehead-slappingly embarrassing to get caught in this trap AGAIN.