Not sure what mill you have and how you are going to install the angle, but you might consider cast iron angle plates. These are already ground and ready for something like this. I used one on the rear of my 80/20 mill build and it was absolutely perfect @ 90 degrees to the table surface. I probably just lucked out, but it worked very nicely. I think the 5 x 4.5 x 6 was less than $20 shipped from Enco. They were on sale.
They still are. Less than $20 for the pair and they already have the holes in them.
Sorry, didn't realize you were across the pond.