Which option for 28 pound sash?

A quick learner from Florida says:
November 9, 2015
Hello. I have identified S380-29 as the correct window balance for my windows. The current balances are stamped "28" and weighing them verified that they are indeed 28 pounds. I am not certain which one to order as 28 pounds falls within the range of both the 2840 and 2850. Is it better for the window weight to be at the top of the range of the 2840 or the bottom of the range of the 2850? Which one should I purchase?

Thank You,
Joe
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Tom from SWISCO responded:
November 10, 2015
Usually in these situations I defer to the stamp on the original balance. Usually it should read something like "28 4" or "28 5." Double check to see if you can make out what your balances had originally. Otherwise, it would honestly be up to you. If your sash is exactly 28 pounds then I see no reason why both options wouldn't work.
29" Series 380 Channel Balance
29" Series 380 Channel Balance
S380-29
$11.10
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