It really depends on the construction of the boot. Things like the thinckness of the leather, other linings like gortex, quality of sole/inserts all influence things heavily.
I'm good with 400 grs. in a quality boot witha sock liner and good wool socks to temps a lot colder than you mentioned. If that isn't enough it usually means the need to move up to a pac/winter boot.
That said 400,600, 800 are all good as long as I like the boot. Never had 1000 yet but at some point it becomes a game of one ups man ship.
