Good job
I went through this learning curve a couple of weeks ago on my rat tanks too.
If you start your cut at about a 45 degree angle on high speed then slow your drill way down after you get your cut in the upright position it will cut great. Just remember high speed creates a lot of heat and heat is not good with glass.
Also i broke my first tank at that "almost there" spot because I didnt realize how "almost there" I was and dropped my drill through the side when the hole was there I wanst expecting it at that point and it is really hard to get use to drilling and NOT pushing on it.
***TIP***
I did find that it works great to back your glass. I set my tanks on some scrap 4x4s that I had and cut from the inside. You still cannot and dont want to push but my cuts came out a lot cleaner on both sides with the wood backing the glass.
Also, if you are not going to use a constant water flow, you might try filling your putty ring with sewing oil because it will disapate the heat better.