Think you're not setting your alarms effectively bud - you should get zero false indication on a tight line.
You get some funny old takes on zigs, if you wait for the rod to tear off you'll miss loads of bites.
You really don't need to know exactly how far under the surface it is. As long as you know the depth (which you should do if you're planning on fishing zigs) just guesstimate the length of the hooklink. A couple of inches here and there make no difference IMO.
If you're getting indication just glance at the rod tip, it'll bounce (at a minimum) if there's a carp involved.