how to correctly measure shaped armhole

[yayaknits]

Using older pattern; simply states cont until armhole measures 15 cm.
Does one measure from lower edge of armhole straight up to top edge or along curve?

[salmonmac]

Don't meassure along the curve. Measure in a straight line up from the initial armhole bind off or decrease. It may be easier to move in on approximately the initial bind off row and measure up from there to the sts on the needle.