I should get more organised and make up a slotted jig for holding the bit, but it'll do for now. I can't imagine that this would take you long on your mill - you'll be able to do in 1 pass what I took 10 to do
One tip about getting the right hole position - I sharpied the side of the block and found a transfer punch the size of the slot. Then I held it in all the positions I could think of using it in (fully down, level, tilted up) and scratched a line the length of the slot each time. Where they intersected is where I punched the mark.