Shown below is a current transformer with the number of turns between taps. In the space provided, write the calculated voltage for taps X1 to X3 when 200 V is applied to the full secondary winding (X1 to X5).
So, you know the turns for X1-X5 is 100. We also know that the turns added together for X1-X3 is 30. Then we apply 200V to X1-X5 and read X1-X3.
So. (100/200V)=(30/X) => X=(30*200V)/100 = 60V
Simply put it is just a ratio of the turns you are measuring compared to the overall turns applied voltage.
Here's a good pdf explanation on CT testing speaking about this specifically:
CT Testing Basics.pdf
another way to calculate
find percent of coils 200v x 60%= 0.6 x 100=60v