What size cone is needed for conical?

I'm doing a conceptual design for homemade conical. My plan is to take an old pony keg (the taller/skinnier style) and fashion a stainless steel cone and TIG weld it to the bottom of the keg. Kind of like what yuri did, just with a smaller keg. This should give me ~8 gallons of fermenting space, depending on the cone.

My question is what is the slant height needed for proper yeast drop of a conical? This obviously would determine my perpendicular height. I figure I can plug the numbers in, run it through a 3 piece roller and butt weld the seam. From there I could tack on a dump valve and a drain valve.

As stated this is a concept idea. I got the keg, and a source for some stainless, and means of obtaining the tools. So I might run with it if I can get all the details hammered out.

Proper angle is 60* for a conical fermenter.

60 is indeed the standard. Theoretically, you could just have a cone if you wanted. It would make for a cool looking fermentor, if not particularly space-conscious.

Yeah, a total cone would NOT be beneficial to me, I don't think it'd fit in my ferm chamber. Wow, I'm really going to have to sharpen up on my trig/geometry because I'm drawing a blank for the formula to calculate slant angle. Is 60 the minimal? Anything above 60 will work I'm guessing, within reason?

After I roll the cone I plan on welding a nipple to the bottom so I can attach a 1 inch ball valve.
This will allow me to keep my cost down for now with a cheaper ball valve until I can afford a 3 piece sharpie ball valve