well now that we know you are not turning it.
Why is it variable? have you been resetting the part on your mill ?
How long is your table on the mill?
I'll assume you have to reset your part 2 times because your table is not long enough.
I would use the slots in your table. put something that fits in the slot (just fits). use a few. use this to set the metal up against (a fence if you will. make sure you have clean burr free slots.. stone first). This is going to act to support the opposite side of the taper. put a piece of ply on the table to be able to cut into w/o cutting into your table. Or use a piece of metal..
Lay the metal to taper so it registers at one end then put a spacer under one of the other base pieces. This is a measured piece for the distance. So if you are 24 inches from the end this would be a .50 spacer. another 24 and it would be a 1.0 spacer lock the spacers down. The lock the metal to cut down.
One thing I failed to mention, your first support must use the corner of the block, so you can slide down and use the corner.. Or start with an offset and add that to each standoff. BTW it would be a good idea to use round standoffs (spacers) .
Now lock down the metal .. cut, move the piece down, reclamp, cut, move recut..
That's my idea, and I'm sticking to it.. BTW if the flat bar is thin and flimsy, you may want to put a backer onto it, so that the spacers register and that holds the part straight.