I think I would do it differently than Chuck. But there are always different ways. If you do it in a vise, work the cut between the vise and the table. Elevate one end, and let the other end contact the table. Use a bevel protactor-level to set the 45°, or an angle block if you have one. Also, on the other end of the vise, put a piece of the same bar (3/4 x 2) if you have it to make sure the vise has a proper bite on the work. Then you can use a regular end mill to cut the 90° slot. This assumes that you can elevate the bar left to right, as in high on the left, low on the right. Usually you will have more clearance on a BP type knee mill that direction. You will likely have to do this with a bit of quill extension, but as always, the less the better.
