Single speed/fixed is meant to be the pinacle of simplicity, dont over complicate things. Stop worrying, buy a sproket, fit it and ride. If you find it too low or high, buy another sproket and try again, unless you run a ridiculous ratio you are unlikely to find the bike unridable, you will make do until you find the right ratio. Its a single speed, if you spin out on a hill, freewheel.
I've cycled up hills like yashicamat rides with 65" with an 85" gearing, its not ideal, but thats the gearing I have/had so I did it. You learn to do that when you have one gear.