I needed to find the relationship between the pinion shaft and the mounting holes in the bellhousing. You're after a simillar figure.
According to Jordy's Skyboat the mounting holes are a perfect circle around the pinion/input shaft. Jordy certainly knows his stuff so take that as gospel.
On the above basis you need to ascertain the diamater of that "perfect circle" and you have your answer. You can't measure between two holes to get the diameter as there are no two holes exactly opposite each other.
I just passed the information onto my machinist and left it to him to sort out.