Thinning behind the edge - how much is enough?

For the first time today I was unable to bring three of my gyutos up to the level of sharpness I have become accustomed to by stropping or using a polishing stone. So I set out to re-bevel them starting with a 400 grit Chosera stone. I recall Dave saying that metal needed to be removed behind the edge in order to avoid a progressively widened bevel angle. I was unsure how far up the blade to thin. I tried to maintain the original angles as best I could. In order to do that I thinned up the blade (closer to the spine) about a centimeter. Is that about right? It was a lot of workhmy: , especially on the blue2 gyuto. The DT-ITK was much easier. Any suggestions would be welcomed.
Thanks,
Jan

I've never been much a measurements guy when it comes to sharpening. Thin it until it cuts well, but doesn't collapse/chip/deform/etc. It's kind of a trial and error with every knife. However, 1 cm is a decently wide bevel and would seem to suggest you thinned the shoulder. I've personally thinned knives resulting in grind marks 3/4 of the way up a 2.5" wide blade until I was satisfied with it's cutting ability.