How to measure an armhole?

[Hscmum40]

I've been making a pullover for myself and now I'm a little stuck. I have to keep knitting until the armhole measures 24cm from beginning.

How does one actually measure an armhole? Do I measure from the needle down to where I started casting off for the armhole? If so do I measure straight down or do I go diagonally from the last stitch on the needle to the beginning of the first row of the armhole shaping? Or am I completely wrong anyway!!!

Thanks

[suzeeq]

Measure straight up from the row you do the first BO on the armhole shaping.