I don’t quite understand what the pattern says ‘cont without shaping until there are 15 rows less than back to beg of shoulder shaping ending with a knit row
Welcome to KnittingHelp!
Continue without shaping means without increases or decreases. You can figure out what 15 rows fewer than the back is by laying the back out on a table and counting down 15 rows from the beginning of the shoulder shaping (likely a cast off of sts). Mark that point with a safety pin or stitch marker.
Now lay your current knitting on top of the back and see how close you are to the marked row. That’ll give you an idea of how many rows you have left to knit on the front. You can always keep measuring against the back to see when you approach the marked row.