It's a hard one to scale but this is how I did it. I concentrated on the neck as this is the bit that needs to be right not the colar. I measured the width of my neck lets say 150mm. I then zoomed on my monitor till the whole pieces width measured 100mm (yes measure the image on the screen with a ruler). Then measure the width of the neck top (it's the thinnest part) on the screen, let's say 60mm. So the neck is 60% of the whole width. Take 150/60=2.5 then 2.5x100=250. So this means the neck+colar should be 250mm in width.
I think that's right 0_o
Hope that made sense,
Tom.