Ligature strangulation: how to know when to tie the reef knot?

[snooperdooper]

So I believe that ligature strangulation is my absolute best method. I am working towards perfecting it but I'm wondering what signs you should look out for before tying the reef knot? That's the point of no return really and I want to be absolutely sure I've hit the no hurty veins and not the hurty ones.