Reibin rails taper .050" in 9.0" which is .050/9 = .0055" per inch

Multiply x 3600 = 19.99 inches at 100 yards

Divide / 1.047 = 19.09 MOA

(One MOA is an angle which subtends an arc whose length is 1.047 inches at 100 yards)

The fully correct calculation is:
sin(angle) = 19.99 / 3600 = 0.0055527777
angle (deg) = arcsin(0.00555277777) = 0.318152366 deg
angle (MOA) = 60 x angle (deg) = 60 x 0.318152366 = 19.08914197 MOA

(Thankyou to Ross Drewe for Trig calculations)