The subject here, as I understand it, is color-blind casting. The basic premise of color-blind casting is that it is not based on formulas or percentages but on who is "best." The challenge this prompts is that "best" is inherently subjective. In turn, this asks what we know about the director and producer to whom you are giving this power. Are they white? Are they white supremacists? Does their orientation bend toward a Western European sensibility? Where is the paying audience and how do they define best. Does a lily white audience in Mississippi "deserve" a white cast because they think white is best?
I could of course let that play out with any race in the driver's seat. However we slice and dice it, this is not a question that admits of simplistic answers.