Have a look at the pictures I posted in these threads.
http://www.cnczone.com/forums/showthread.php?t=39471 http://www.cnczone.com/forums/showthread.php?t=33866
The holders are just blocks of cold rolled steel machined to fit in the turret with mounting holes drilled and reamed from the chuck so they are on center.
If you machine the holder with a step that comes against the upper part of the turret it will always go back in to the same location so all you need to do is make a record of the X position when the mounting hole for the drill shank is done and you have the center position.
After drilling just tap setscrew holes in the top.
I have to admit that these holders aften do not go back perfectly on center but all this does is make the drill rub a bit on the side as it gets deeper. If you are trying to drill a hole to +/-0.001 in size and parallelity this can be a problem but trying to drill to really fine tolerances is not easy, that is what reamers are for.
Holding a 1/2" shank drill in a collet is possible but dubious especially if the cutting diameter of the drill is much larger than 1/2" because the torque needed can spin the shank in the collet, especially if the drill grabs.
For getting boring bar holders on center I put a piece of 1" round bar in the chuck and another piece of round bar in the holder. Bring these together side by side and use either a bit of paper or a feeler gauge between them. The center position when the feeler gauge is just nipped is more negative by the sum of the two raii and the feeler gauge thickness. With care you can get to within less than +/-0.001".