Optimum String Gauges

[David Trainer]

Dear All. 

Is anyone aware of a software tool or calculation method that tells us what the optimum gauge of string [nylon or steel] we should use ? I have always purchased standard sets of strings where the gauges are already defined. 

The main reason for asking is I have an old 8-string mando-cello which has been butchered by the previous owner and now only has 6 tuning pegs. I was thinking of trying to set this up as a 5-string cello banjola tuned an octave below the normal banjo tuning. The mando-cello has a scale length of 58cm and obviously no short 5th string. 

How would I calculate the best string gauges if I were to use nylon ? and how would this change for steel strings ? 

This might be a complete waste of time, but it would be a shame not to try. 

Any thoughts ? 

Thanks. 

Dave