745 rear sight on 645

[Yendor357]

I know this has had to have been covered numerous times in the past, but I’m late to the party. I used the search feature. Like usual, I had no luck. So apologize in advance for asking…..

I ordered a 745 rear sight from Jack First.

The question: From top to bottom, on the raised rib behind the rear sight, How far down do you mill? Do you just stop at the base of the rib?