Hello!  I ran the Neutral Grounds Casefile for a group this weekend, it was my first time as GM so we spent a lot of time hashing out how certain rules worked.

We had an issue with the landmine encountered in Act 2.  Here is how it is described in the file:

He’s surrounded by a hemispherical “land mine” type ward—it’s a 6 shift ward that, if tripped, explodes into a 4-shift attack (yes, that’s a +4 attack at Weapon:4) on everyone in the room, including Frank.

I took this to mean that I would roll, modify it with the 4-shift power of the attack, compare with the players defence and then add the weapon:4 to determine the number of shifts the players would take if the attack succeeded.   My players rolled pretty badly, resulting in 10 shift attacks on two of them and 8 shift attacks on the rest.  It seemed a bit extreme for that early in the game, so I ended up reducing the damage since I didn't want to cripple them going into the last act.

Was that the correct way of calculating damage for a landmine type ward? 
Should I even be rolling for the attack, since as it is a thaumaturgy effect, it has a set amount of power in it?  Or would they just roll their defence versus the +4 attack?

Thanks for your help! 

At least they're going to be a bit more cautious when investigating from now on.