My butcher will happily mince me some (grass-fed!) beef at whatever ratio I ask.
I'm thinking 50/50 but am not quite sure. I would love to hear from people their preferences or recommendations and reasons why.
Should I ask for any particular cut of fat or lean to mince?
From a purely "cooking" perspective, I can't think of many applications for ground beef any fattier than 70% lean. Most uses for ground beef in modern "Western" cooking will call for 75%-85% lean. You'll probably just end up scraping up a lot of leftover fat from the pan after browning anyway, so I'd just cut the middle man, have him add less fat to the grind, and maybe render the leftover bits into tallow.
I personally like 80% or 85% lean, depending on whether it's getting sauced, slow cooked, thrown into chili, etc.
(I'm not familiar with the minutiae of Paleo cuisine, so there may be some narrow application for super-fatty ground beef of which I'm unaware, but nearly every ground beef recipe I've ever used or devised requires browning the meat, so one way or another a lot of the fat ends up liquefied in the pan anyway.)