I am trying to get the hang of FG and while doing so I used a Cleric spell "Command" against a Goblin
[CAST] Command [at Goblin 8]
[SAVE] Wisdom
Save [8][vs. DC 13] -> [for Goblin 8] [vs Prällen] [FAILURE]
The cleric has wisdom 16 that gives a +3 bonus
The Goblin has wisdom 8 that gives a -1 bonus
So in the above example the Goblin rolled a 9 - 1 = 8
But how was the DC calculated?