Others may step in here and fill in where I am missing, but the trigger wire is just that... It turns on the transistor. This is why you want it ideally to be of smaller gauge than the power coil. I know of others who use a very fine gauge ( see the Daftman videos on youtube )

Ideally you want the ohms of all the power coils the same.

What I have found helpful is to do what Erin does, is to go to a place outside that is flat and mark off a certain distance. Then make all of your power coils this length. I did not get this far, but I believe if I filled my coil all the way, then took this outside and unwrapped it, then simply double it back that would give me the length of coil that would fill the coil for a bifilar.

I hope this helps..