Hi C string

[guitar344]

What is the longest scale length I can safely tune say a 9 up to C5 C above the hi e string. I know it's impossible on a standard 25 scale. 21 is pushing. 20 might be a bit of a stretch.

[guitar344]

https://wahiduddin.net/calc/calc_guitar_string.htm Did some calcs. A 9 tuned to C5 would be at 80% of breaking tension on a 20 scale. A 9 on a standard 12 string assumng 25 scale tuned to G4 would be at 70% of breaking tension.