Measuring armholes

Newbie needs help please.
Pattern says “cont even until armhole measures 8 1/2 ins”

I figured out “even” means no inc or dec, but I don’t know how the armhole is measured. I’m doing a sweater vest which starts with the back and have been shaping the armholes in for about 16 rows.

Hi, welcome to KH. Assuming your pattern told you to bind off some stitches, you would measure from the bind off. HTH

Welcome to KH!
Yes, from the bind off and measured straight up. Follow the row of the first bind off (16 rows ago) across into the body of the sweater and measure straight up from there. If there’s a schematic with the pattern, it should show this measurement.