til out vinyl replacement window channel balance from Atrium window

Thank you for your question. At first glance, I am led to recommend the S775-25 channel balance. Seeing the way you measured the length, it seems you stopped at the top of the plastic attachment. You want to measure the length, including that piece but not the attachment. It should measure 25. Your balance should also have a stamping/marking on it to determine the stamp option. If there isn't any stamping, then you will need to weigh your sash on a scale and refer to the sash chart on the store page.