I am installing a new toilet for a basement bathroom. Rough-in toilet closet flange is PVC which wasn't level. I installed tile and am using a PVC spacer ring, one edge is slightly higher on one side above tile and about 1/4 inch higher on the other side. Hopefully this is acceptable and wax ring compresses without any problems to form a good seal. I need to fasten spacer ring to closet flange. (I'm assuming closet flange is glued to plumbing below the floor in the concrete, it isn't flush with top of concrete.) So I'll use silicone to seal spacer ring to closet flange, and concrete screws through top spacer ring into the concrete below. Are Tapcon concrete screws the fastener of choice here? Will the standard Tapcon screw be corrision resistant enough? Also how many screws do I use in the 8 holes on the flange, all 8? Finally, do i use 3/16" or 1/4" screws? Thank you.