John,
For what its worth department; two full threads, three you take a severe risk of hitting the glass. We-I have broken/cracked a windshield because of the uneven edges of the glass!
When the shop was open it was our procedure to remove the entire windshield from the OEM frame and actually grind*/sand the edges smooth then re install.
*using a 6" grinder with a 24 grit pad, and applying at a large angle to the glass we were able to actually grind all the edges smooth. Never had one break since the above was implemented. Go slow....a Valium for nerves
, As little heat build up as possible! (How we shape all of our "laminated safety glass" windows in our street rods, I.e., chopped tops, etc.)
DV