How do we calculate the amount of armies worth saved when you throw a mortal on them? Do we multiply the amount of armies with the percentage of it hitting?

The expected armies saved by a mortar is the armies on the territory x the percentage it removes (normally 20, but that can be increased) x the chance of it hitting. So targeting a 1B territory with a 70% change to hit is 1B*.2*.7=140M average armies saved.