Correct channel balance length

A quick learner from Review says:
I need to replace two channel balances in a large kitchen window. I think someone installed the wrong length in the past. I've identified your 770 series as the proper balance, but what measurement do I use to select the correct length? The window, from the top to the top sash on the lower window is 38 1/2".
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Paul from SWISCO responded:
Thanks for posting. Our customer service tells us that you reported a visible glass height of 37" and a sash weight of 25-30 pounds. With this in mind, please see our S770-38 channel balance. I'd also recommend the 38D stamp option.
How to replace a tilt-window channel balance.
How to replace a tilt-window channel balance.
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