I traced the outline of the hole from my bulkhead using a sharpie. I was drilling from the outside of the tank so I taped the inside of the tank (to prevent the glass circle from falling and cracking the other side of the tank once its cut out pretty much). Its late and I 'm guessing, but I think the dremel bits I used are 7103 and 7134. One is a ball point that I used to punch through at one point in my traced circle, and the other is a cylindrical shaped bit that I put inside the punched hole to drill out the rest of the circle. I bought both bits at walmart.
I just did a quick search and it seems alot of people recommend using the hole saws over dremels on thicker glass which I'm thinking you may want to check out. I used a dremel on very thin glass on 5-20 gallon tanks. Apparently thicker glass takes much longer and burns up dremel bits quickly but I have no experience with this.