Spiral balance does not fit

A quick learner from Acton, CA says:
I received the S900- 27" spiral balance for my Better Bilt Window and it seems not to be the right size.
The window is single hung 54 high X 34 wide and the lower window weighs 18 pounds. For the measurement, I measured from the top of the window to the top of the lower window when closed. 27" is what I came up with and the website said it would be a blue tip, not red.

I installed the balance with very little tension on it and tried to lower it. It stopped short of the bottom of the sill by about 2 inches. Pushing it caused the spiral to rip out of the tube and destroy the whole thing.

What is my correct measurement so I can order the correct length and tip color?
​Please let me know as soon as possible as I have to vacate the house Friday and I don't want the new owners giving me a headache.
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Tom from SWISCO responded:
Thanks for posting! I'm afraid the method you used to determine length was not accurate. We usually go by the length of the tube for the old balance, but assuming your old hardware is missing you will need to give us the height of the visible glass for the sash that this balance supports. We can use that to calculate the best length. Please also give me the weight of the sash so we can recommend the best tip color.
A quick learner from Acton CA says:
I gave those measurements in my email.
This is a single hung window
Total Window height 54"
Total Window width 34"
Bottom sash:
Height 29"
Width 33"
Weight 18 pounds.
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Tom from SWISCO responded:
Thanks for getting back to us. I believe you gave us the total sash height. We specifically need the height of only the visible glass for the sash in use. This is what we use to calculate length. Let us know whenever you can.
A quick learner from Acton CA says:
The measurement of the visible glass in the lower sash is
30 1/4" wide
26 3/4" height

the weight of the lower sash is 18 pounds
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Paul from SWISCO responded:
Thanks for the additional information. I would recommend a 29" balance, since the 27" didn't work out for you. I would also recommend the Red Tip. Before you install the balance tube, have the spiral fall out to make sure it touches the sill before screwing it in. That's a good method of determining whether or not it is the correct size. Also make sure to charge the balance during installation. You can see our guide here.
A quick learner from Acton CA says:
Thanks. I DID let the spiral fall all the way out and it touched the bottom of the window.
I then pushed it back up inside and then let out about three spirals before I attached the blocks. After re-attaching the window to the blocks, it went down to about 2-3 inches from the sill and stopped. There was no tension felt at the bottom but when I pushed a little harder to get it to fully extend, the spirals blew right out of the cylinders.
I will order the longer one with the red tip.(29")
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