well if you were dumb like me and got the spigot with a 90 degree spout facing down.. then that pretty much dictates how far up its going.. if you got the straight out spigot.. then as low as the nut on the inside will allow.. remember you have to be able to tighten it from the inside.. so as long as the nuts in the inside is 1/8th off the bottom your good.. guessing you wont be fermenting in the saem bucket so trub isn't an issue.. get it as low as it will allow you
edit: the 90's are better for bottling tho.. so what i do is just turn a little after use.. because my spigot goes lower then my bucket.. so if its not hanging iover a countertop it would be resting on the bottom of spitgot.. but they turn pretty easily.. just make sure its tight when you send 5 gals of delicious beer into it..