Front sight slots on barrels

[rifle]

Any machinist types know what the proceedure is for setting up to mill the slots for blade front sights? The machinist proceedure?
The proceedure to have the slots centered and perfectly vertical to the guns line of sights?
I have ways to mill dovetail slots for the dovetailed front sights and get those done well but my proceedure isn't of the standard machinist-gunsmith  proceedure. I haven't milled slots for blade sights very often. I lost (was stolen with some cutters for the mill) my woodruff key cutter and don't know the size I had. I used it for cylinder notch repair after welding and for slots. Not too many times for slots for front sight blades.
I figure there are machinist-gunsmith types here on this forum.