Yesterday evening I finally got round to checking the pattern sizing against a tried 'n' tested pattern, doing the forward head and rounded back alterations on all the front, back and facing pieces, laying out on my fabric and cutting out.
I add a wedge across the back which adds an extra inch. And I also take a little bit off the top of the centre back seam. This is a super easy alteration on things which have a centre back seam.
This will then need a matching forward head alteration to remove the excess from the front neckline.
Again draw the line at 90 degrees to the grainline. Then mark 1" up from the line and draw a second line at an angle back to the hinge point in the armhole. Fold so the two lines meet and tape in place. Alter any facing and collar pieces to match (easiest done by transfering the two lines onto the facing piece and then folding afterwards).
This leaves the armholes and sleeves exactly the same and thats not been a problem for me.
I thought for ages I needed a small bust alteration but I don't - the excess fabric for me is between the bust and the neck so this is where I need to remove it.
I tend to pick styles with scoop or V neck as I don't like the sensation of a neckline at the collarbone level. However if you do have a higher round neck style and do this alteration, you will need to redraw the front neckline slightly lower, and alter any facing/collar pieces accordingly.
I hope this helps!