Thank you very much for that Robert! The numbers seem to work out and the math makes sense to me.
A key factor appears to be to calculate the focal length of the lens BEFORE calculating the angle of view. Having the correct figure for the speedbooster and Sensor size is also handy
12mm x .64 = 7.68
AOV=DEGREES(2*ATAN(SensorHoriz/(2*FocalLength)))
AOV= 2 x ATAN(15.81/2 x 7.68)
AOV = 2 x ATAN(15.81/15.36)
AOV = 2 x ATAN(1.029296875)
AOV = 2 x 45.827
AOV = 91.65