Hello,
The following explanation assumes you have or you can create a frame at the V-notch tip with the Z axis pointing up.
z - is your sensor measurement in the vertical direction relative to the above frame;
R - is the radius of the cylinder;
H - is the distance from the tip of the V-notch to the center line of the cylinder;
a - is the half angle of the V-notch (e.g. 90/2, 30/2, 120/2);
(1): z = R + H
(2): R = H*sin(a)
Substitute (2) in (1):
z = H*sin(a) + H
Then:
H = z/(1 + sin(a))
From (1):
R = z - H
I have a picture for it, if you need it please let me know where would you like me to upload it to.