Finding balance length from existing overall sash height.

A contractor from Panama, Central America says:
I am servicing a few dozen PVC single hung. Some of them are without the block and tackle balances, which I have identified are similar to your S770 series.
I do have the sash overall height and the weight.
If the sash height is 34 inches, what would be the balance length?
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Casey from SWISCO responded:

Hi there, thanks for writing in. We typically determine the dimensions of replacement hardware based on that of pre-existing parts. However, I'm going to have one of our senior experts look into this and see if they can deduce anything. We will get back to you shortly! 

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Casey from SWISCO responded:

Thank you for your patience in this matter. For most vinyl sashes, the glass height is 2" less than the overall height of the sash. The balance height is based on the glass height, which would put you at a balance length of 32 inches. The other piece of information you will need to select the correct balance is the sash weight. Once you have this, you can consult the chart on the product page to select the correct stamp option. 

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