Most rotary or indexers have between 72:1 up to 100:1 reduction.
The diameter of what you are working on is really big however Tormach makes a very inexpensive eight inch CNC rotary table. You could make a base plate that would raise the rotary up 25 inches and use a adapter fixture to mount your part to.
After you finish running your job you could sell the rotary for a slightly discounted price, everyone here is always looking for a deal!
Good luck with your project.