Measuring the centerline distance between 2 adjacent studs I get 2.625 with a scale. The formula for finding the diameter of a circle when you know the chord length is chord=d sin(c/2) with c being the central angle, 72 degrees in this case.
2.625= d sin(36)
2.625= d (.5877852)
divide both sides by .5877852
4.466=d= your calculated bolt circle
Conclusion: My calculations are 0.034" off of 4.5" so given the margin of error in my measurement of the chord using a scale, I conclude that your bolt circle is 4.5"
Sorry I couldn't respond earlier but work and family got in the way.
2006 23B Hybrid with 10k round bar WDH
2011 F150 4x4 SCREW Ecoboost, Max Tow, Integrated TBC, 3.73 LS axle, Firestone Ride Rite airbags.