looking at your diagram, look at the ‘green’.
Its the angle in excess of 90 degrees. So a proper measurement would be in the range of 0-10 degrees.
Cant can perhaps be described as a fin leaning toward the closest rail
and is the angle in excess of 90 degrees leaning towards the rail.
Alternatively a protractor could be put to any side of the fin to measure the angle off of the perpendicular(90deg.)
A fin leaning towards the rail would have a positive cant of 0-10 degrees.
A fin perhaps leaning inward towards the stringer could be described with a negative number of 0-10 degrees.
A fin straight up and perpendicular to a flat board bottom has 0 degrees of cant.
All measurments should assume a flat bottom or close to that even if there is considerable concave present.