I've currently signed up to the Craftsy class 'Sizing Knitwear Patterns' by Faina Goberstein. It covers swatching and working out the gauge and also helps to work out the pattern for different sizes.
It's a great class and shows how much work goes into preparing a pattern.
Whilst watching this class I decided that it may be best to make the gilet as a pattern only and not sell the actual finished piece, at least not yet. I thought that if I focused on pattern writing it would give me a chance to improve as a designer and would mean that I would have the finished piece to wear myself without having to make another one.
I may also rip back the gilet, as my initial swatching was somewhat rushed. However, as it's a simple design I'm likely to continue.
When I start the next project which will be a knitted ruana or stole, I will definitely take the time to do things properly.