The taper to be measured is in blue color.
Let the yellow and orange tubes contact with the taper to get A dimension (distance between two faces).
tanα = ((D2-D1)/(L1-L2-A))/2
α: haft taper angle
D2 and L2: outer diameter and length of the yellow tube
D1 and L1: outer diameter and length of the orange tube
In case of go-no and go control the red area on the scale of the orange tube should be used. It is determined according to the tolerance of taper angle α.
This measurement is faster but less precise than other known method (using balls)