You don't need to use a calculator to work out duel gear, and I wouldn't really trust one anyway. They'll always give you weird combinations when all you need to do is wear full Cullen's set. Some people like Doolin's, but as Charisma skills are almost completely useless, I think that set's overrated. Still, if you can't get your hands on Cullen's, it's an acceptable alternative. You might be able to beat Doolin's set with a custom selection of gear, but probably only at high levels. I haven't tested custom gear sets since before Cullen's came out, so I can't tell you for sure.
Same goes for fort fights with Deadwood's or Black Bart's. Their total AP/SP bonus is pretty similar, so just get your hands on one and assign your SP around it. If you're not a specialised fort fighter and are just looking for a set to wear when you do happen to attend a battle, Deadwood's is better since it gives HP.
Clothcalc is easily the best calculator we've had over the course of this game, but you can't really rely on a calculator for activities like duels or fort battles since they're not always a simple matter of having the most overall bonus SP and there's a certain amount of personal preference involved.