These all have cinches in the back, like most any dress waistcoat/vest you'd be familiar with, so that does allow for some adjustment in the size at the waist. If you are looking for a "magic waistcoat" that will be guaranteed to fit no matter your weight change, I can't promise that! But it will fit the measurements you provide on order, and should still do you ok a couple of inches either way. Beyond that, it's either a sit-up regimen or a new wardrobe! :-)

Yes, one meter of double width tartan is good for a bias-cut waistcoat.