I shoot a Tornadoe. I have used a Nikon BDC type cross bow scope for 2 seasons. I like it a lot. Has held zero and the "DC" marks are pretty darn consistent as they are described in the Nikon literature.

I had had a 3- red dot, lighted type scope on it before and for whatever reason I have done a much better job of bolt location with the Nikon BDC. Plus one less thing to be concerned with .... don't need battery life !!