Kniting help needed!

[Tatties]

I have been naughty and started to knit a baby hat without checking the gauge. The base circumference is about 40cms... how far should I knit before I start decreasing? I am guessing about 9-10 cms? Is there anywhere I can find a formula for working out this kind of thing? (Am I making sense?? )

I knitted one recently
I don't think there's a formula - but you have to take roll into account if it's st-st
I think the pattern said knit 9.5cms so you are about right

Oh fantastic, thanks
There is no roll at the bottom

Ok then perhps not so long?
9.5cm was for a hat with a roll brim

I do about a third, and then a slow decrease. I do a lot of hats.

You want the decrease to start out slow, and then get faster, iyswim.

Wait, wait, if you're doing it without checking the gauge, how do you know how big around it is? Or have you measured it?

I always base the circumference on a (slightly) stretched circumference.