I used to install stuff like that almost 40 years ago, my uncle did all the heavy thinking.
I did find this:
Sectional Garage Door Extension Springs
Sectional Garage Door Extension Springs
Quote:
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On sectional garage door springs this is normally 15 percent. For example a 100 pound pull spring requires 15 pounds of weight to separate the coils; when a door is open the two springs will support 15 pounds of door weight without any stretch.
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The springs we used ran across the opening, the ones in the article are different.
I suggest you calculate the weight of your door and purchase locally with the advice of the installers/store keeper.
I suggest a mom and pop operation and not some franchise or chain store. Talk to someone with a lot of gray hair or no hair at all.
You might even get a good deal on something refurbished.
They might be able to sell your used springs to someone else, ask for a small discount if you can turn them over.