dob altitude disk size

Hi,

I am about to build a dob mount for a 12.5" f/4.7 tube newtonian. What should be a good altitude disk diameter for this scope size ?
Thanks,
Andre

The rule of Thumb when using teflon and some type of laminate as the bearing materials is to use the same size bearing as the size of the mirror. So a 12" mirror gets a 12" bearing. But really there are so many variables. Several years ago Sky and Telescope reviewed 8 or so 8" Dobs, and the Orion with a small nylon bearing and a spring actually moved and balanced loads better than a premium Starsplitter Dob with the traditional large bearings Eboney Star Formaica. Still if possible, try to use the largest bearing possible that still looks nice too. Just my 2Cents.