There are a ton of beer line calculators etc... that will all give you different numbers. Many of us just use 10' lines as a minimum. It's a good overall compromise and works with many different beers.
not 100% on this, but a good rule of thumb is 1 foot of line for every 2PSI you are serving with.
For example: serving at 12PSI on your regulator, needs at least 6' of line.
This is just minimum, and as stated above, many people go with 10' and call it a day.
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