Measuring armhole shaping

This pattern is written a little different than other ones I’ve worked. Others have it written, decrease…work til back measures … inches.
This pattern says work until armhole measures 5 1/2 inches. My question is do I measure the length on the diagonalhan or straight down to the start of the decreases? There is a big difference in measurements.
The vest I’m working on is a little boys vest so that’s why the inch length is smaller.
Thanks.

Measure straight down from the shoulder, rather than the edge of the armhole.