Determining length of Series 900 Spiral Balance

A quick learner from Spiral Balances for Tilt Windows says:
The tip is red and the diameter of the tube is 3/8". When I measure the tube it is 28 1/2" long. I'm just measuring the tube. I am not including the spiral or red tip in the measurement. The closest size to the existing tube seems to be either 28" or 29". Which size should I buy or am I measuring incorrectly?
Thank you for your help!
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Paul from SWISCO responded:
Thanks for posting! Well, first I would make sure that you're just measuring the metal part of the tube. We do not incorporate the plastic tips and spiral protrusions in our length dimensions. If it is, indeed, 28-1/2" long, then I would recommend our 29" Series 900 spiral balance, or our S900-29, since our balances are only manufactured on the inch.
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