35" window channel balance help

The 35" measurement is good. You'll need a 35" balance. If you've got a 15-001 shoe (on the right below) and a 19-002 top, then you will need the Series 390, specifically the S390-35. If your shoe is different, and it measures 1" like the shoe (on the left below), you will need Series 351. In either case, I'd expect your stamp option to be 3440, like the stamp in your balance. You can check that by weighing your sash, which should come out to a weight between 23 and 30 pounds.