I am having some discussions about where to put the datums on the attached picture so that we can accurately measure the two circled features on a CMM.

The bottom could be datum A, but the next datum is being "debated". I suggested using the hole pattern as Datum B, but am not 100% sure how to explain it so that someone with limited GD&T knowledge would understand. There will also be a language issue between the designers and the fabricators.  US - China

Any help would be appreciated.