Ok, i cannot stand it any more, here's how i cut a fork ONCE in my life :
(a) build up the bike with fork and headset and WITHOUT the lock ring and use the built-up state to mark the fork where it needs to be cut. you cannot beat this technique for precision.
(b) get two small 2x4 blocks and clamp the fork horizontal in a vice. position the 2x4 blocks so that they END right where you should be cutting the fork. rest your hacksaw against the side of the blocks and use both blocks as a guide while cutting your fork, to match the markings you made in step (a).
(c) use a file to clean up the threads from the inside, testing the lockring liberally to make sure it threads easily. then reassemble the bike.