Thread: Barrel threading for suppressor

I am going to buy an ASE SL5i suppressor for my Tikka 260, I'm thinking of going for a M14x1 thread.
Am I right in thinking that you require a 1mm shoulder for the suppressor the tighten against?
My Tikka muzzle has a diameter of 15.6mm so an M15x1 would be a little too big?

Also, can anyone recommend somewhere in the Waikato or Auckland where I can get my barrel threaded?
Don't want someone that is going to stuff up my rifle, but some places seem to charge up to $130 which seems pretty steep.

Cheers