I have a non-Xcel spreadsheet which I've used for years. However, here's a quick method.
https://www.stewmac.com/FretCalculator.html
The relevant pieces of knowledge I would use for a "highest pitch before breakage" is that a steel string will break a bit beyond G4 at a 25.5" scale length. A guitar's high E4 string is in safe territory, and can even go that 1.5 steps higher.
And that means one can tune a plain steel string to G5 at a 12.25 scale length.
So, using that helpful calculator from StewMac, I can figure out the fret offset distance from the 25.5" nut.
Here's an example from real life:
I see a five-course mandolin/mandola for sale, with a 17" scale length. What is the longest scale length I can use to reliably tune to high E5 for that top course?
So, using the StewMac fretboard calculator, I enter in 24 frets, and a 25.5" scale length.
Now, counting up the frets and incrementing my pitch of G4 by half-steps with each fret, I see that I get to E5 at the ninth fret. That fret is 10.338" from the nut. Subtracting 10.4" (rounding up just to make it easy) from 25.5" leaves a scale length of 15.1" to be safe.
So, I now know that 17" is 1.9" into breakage territory. It might last a little while, but not for long.
In this example from real life, I actually contacted the seller, and the seller indeed verified that the top course had never lasted over time tuned to E5. I saved myself a bunch of money, as I wanted an instrument in standard mandolin/mandola tuning.
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It took a lot of empirical experimentation over many years before I accepted that weirdly converging limit on the pitch of a plain steel string. "I'll try a thinner string! That'll work!" Nope! *laugh*
Good luck!
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