Vest front

Knitting up a vest and here are the instructions for the front:

work as given for back until 20 rows less have been worked than on back to beg of shoulder shaping.

So, looking at the back of the vest, starting at the point where I started shaping the shoulders, I would count down 20 rows from that point and that is how long the front of the vest should be?

Also, how do I go about counting rows? This vest is worked flat in stockinette stitch. Thanks for your help!

You could count down 20 Vs on one side or 20 loops on the reverse side. Then just measure the length in inches from the beginning of the back until the 20 row point. That measurement is how long the front should be before you begin the next direction.