While there is nothing cast in stone dictating pin depth/exposed, common sense should be your guide. Obviously, having more pin exposed than you have epoxied into the butt is a bad structure. In certain circumstances the exposed pin can act as a lever on the portion that is installed in the collar area. The longer the lever, the less force that is required to move it and the installed portion.
Pin length is up to the builder. Be it joint pin or 'A' joint, pin length can be varied to 'zero-in' to a particular desired ultimate wght. of the cue and it's corresponding bal.point. With regards to joint pins, I've adopted a policy of 1/3 exposed, 2/3 installed. My pin of choice is the 3/8-10, flat-faced with an exposed length of 1 3/8". Do the math.
'A' jnt. pin length, again, is up to the builder and how he wants to manipulate wght. Given the design of my 'A' jnt., minimum 'A' jnt. pin length is 3 1/2" and can be as long as 5 1/2" as the cue requires.
It should go without saying that I'm a big fan of forward weighting. I appreciate the fwd. wghtng. of a steel jointed cue but don't like the 'hit'. I believe I've accomplished my desired result by keeping the wght. on the inside of the cue rather than on the outside. I also like that the installed wght. is in front of my grip hand rather than behind it (as with a wght. bolt). Solid structure is key to a great hit.