Inductor ground, where to put it
Actually, you can't detect current in a wire by wrapping another wire around it. The magnetic field of the first wire doesn't couple to the second wire. If you think about it a bit, you'll realize that they're essentially at right angles to each other. Wires need to be parallel in order to couple inductively.

If you want to pick up the spark pulse inductively, as many timing lights, etc. do, you'll need to put a ferrite toroid around the ignition wire, and then wrap your sense wire around the ferrite (by passing it multiple times through the hole), too.