Liam,
I’m not sure if this is what you’re referring to, but here’s a widely used string tension calculator:
http://www.cs.helsinki.fi/u/wikla/mus/NewScalc/
String tension calculators like this, as well as the actual formulas behind them, are readily available on the web-
The question of arched versus straighter neck – I was under the impression that one of the reasons for the harmonic arch is to reduce the length of the strings in the higher ranges – as a result, you can use a lower