In Dom2, I set just about every province to 11 defense. In Dom3 I have found 21 to be reasonable. Like Gandalf, I too compliment trouble spots with a local commander or two. In strategic choke points (if the gold is available) I crank it up to 30+, but that is rare and usually pretty late in the game.
High PD for MA Ulm has never let me down. Although in my current Pythium game Mictlan of all nations routinely rolls my PD of 60+ and Oceania too. Makes me wonder if the money spent is worth it. I think out of maybe 15 attacks per turn to provinces undefended except for PD around 60, 2-3 are successful.
you should only build pd's at choke points and even then it's best to pad it up with one or two battle mages. I use pd often as chaff and anti call of the wind forces, nothing more.