First time knitting sweater

This is my first sweater/jumper. I have completed baçk and armholes etc but havin completed the front I have come to armholes and the pattern says to complete as per back from shape armholes until 26 less rows have been worked than on back to shape shoulders. Now the back has : shape armholes … Dec rows … (I get this bit) continue straight until armholes measure 21cm.

So how to i work front 26 rows less when I don’t have the number of rows for back just a length??

Many thanks

Nikki82

Welcome to KnittingHelp!
Fortunately you have already knit the back of the sweater. You can count down 26rows from the “shape shoulders” on the back and then either mark that row or measure or count rows up from the initial armhole bind off. That’ll tell you when you have 26 fewer rows and can go on to the next step for the front.
You could also convert length and row number by measuring the row gauge that you’re getting on either front or back.
Good luck with the sweater.