About rotor phasing, since the rotor and reluctor are situated on the top hat and are not adjustable being fixed to each other, the trigger point is by the pickup in the distributor body. To change phasing, you need to alter the pickup to distributor body, that references the cap. The cap locator is often tab at vacuum advance unit, or plate on lean burn distributors. But the cap clips allow very limited adjustment. The vacuum advance plate can do that, moving pick in a sliding fashion, with a means to move and lock down. I have used a longer screw in the capacitor hold down, on points distributor for that. Or drill and tap plate at similar location.
So if you inclined to worry about the end of slot, to weld on, it is not good logic, because phasing is not set there unless you break, and modify reluctor to rotor position. An example of that is the BB flip that moves the reluctor a tooth width, for CCW rotation, placing trigger in correct trailing tooth edge.