Hi Daz,
If you do adopt the square bar method, try to get the hole down the middle of the square. Tap the square firmly down on the base of the vice, do the first flat and then set the Z axis dial to zero. As you mill the other flats, again tap the square bar down, so that by working down to zero on the dial, you cut to the same depth on each side and should thus end up with a pretty accurate square end on your round stock.
For the 45 degrees you mentioned, you would need to make an octagonal bar (or maybe just put a substantial chamfer down two opposite corners of the square one).
Andy