Alrighty, got my calipers out.
For standard g tuning, I recommend using a set of nylgut banjo strings that are made for g tuning.
I provide strings that are made from nylon classical guitar sets. Mainly because these strings are much cheaper, last a long time, and sound nice on the kits.
E tuning is recommended for most of my kits- this is a few steps below g. The guitar strings like e tuning, so I work with that. We take the top 4 strings from the guitar and add another top string or simply use a 50lb fishing line so you don’t have to buy extra guitar strings. Heavier fishing line can be suitable for the other strings. I think it’s best to use the guitar string for your 4th banjo string though because this string is wound.
4…silver wound 0.035”
5…50lb line, 0.025”
For the low octave tuning or “cello” tunings we are using the bottom five strings from the guitar set. An octave below standard g is a bit low and the strings don’t have enough tension to intonate very well, so I say tune up one step from that to low A.
5…0.030”(you could use another 0.040 guitar string here if you want).
If you can find strings +/- 0.001” or so you should be fine.
Please don’t feel limited by your strings. Try different gauges to find the sounds/tuning ranges you are looking for.