Here is a link to help calculate the interference between shaft and coupling...

http://www.tribology-abc.com/calculators/e3_8.htm
It looks like a typical interference ratio... delta/d... is about 0.001. The units in the article are those silly metric things.

Anyway, a shaft of diameter 1.25" should have an interference delta of about a thousandth of the shaft diameter, or about 1.25 thousandths of an inch, or 1.25 thou" as it is written sometimes.

I am not familiar with the surface finish arguments there, but it looks very much like you have consider them also.

Still, it gives an idea of what the designers have to consider.

Rockter.