I do a similar lead-in on a mill on my heads. But If I were to go straight in at 90* like you show I would hit water on my crummy old Ford junk, so I angle my mill head at 15* so as to gently taper it toward the rough port casting wall and minimize the possibility of too thin walls in the roof of the ports.
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FORD 300 inline six - THE BEST KEPT SECRET IN DRAG RACING!