How to detect a drop in mana using variables/triggers?

Okay, if I wanted to detect a specific amount of mana loss from a hero, say 150, to make a trigger go off, how would I go about doing this? I know (or think) it has something to do with variables, but I'm not exactly sure what value to give them.

Anybody out there know?

Have a look in the depository & tutorials ?

There lies some method to use mana drops detection to see if a spell as been effectively cast.

Regards

It's very easy just use the event: unit-mana