How to decide sash balance Weight

A home owner from Southern California says:
My old sash balance show: 25 5 BSI TA or BSI PR different windows,
My sash weights 26 lb.
I don't know I should choose two 2420 or 2450
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Casey from SWISCO responded:

Hello, thanks for writing in. I see that you were looking at the Series 380. I can't confirm if this is the right series without seeing the plastic attachments, but if you are confident that this is correct, then your stamp indicates that you would want the S380-26. With this series, the length is typically one digit higher than the stamp. If you go to the product page for S380-26, you'll see a stamp option for 2550, which indicates a sash weight between 25 and 28 pounds. This would be the one you would want. Please let me know if you have any further questions.

A home owner from Southern California says:
Thank you for professional answer, the size is 26",before I think it is 25".i worry about weight,it is around 26lb, but before I never can close window whole the way, it always leave a half inch open. I think if I choose light spring, it may work better.
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Casey from SWISCO responded:

We typically don't recommend straying from the spring strength indicated by the stamp. If you're window is 26 pounds, then the 2550 option should work well for you. Choosing a lighter spring strength could result in the window not staying open. 

S380-25 : 25
25" Series 380 Channel Balance
S380-25
$9.64
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