Hi all. I previously made an underquilt from a cheap sleeping bag and now have the bug so now Im going to make a pertex/primaloft Cocoon. I decided on going for the Primaloft Sport and done some calcs based on the 1" per 20 degrees below 70 F, assuming this is the comfort rating which Im pretty sure it there and there abouts is, what comfort rating would people aim to get down to? Currently I was thinking -6C (62.5mm) but would this be over the top for the UK? Also this would make the materials very expensive.

I know its a bit of a what would you use it for question but I dont really know as I dont seem to get out in the woods enough. So if it was you, what temperature would you want it to go down to and when would be your primary use time?