2) Put it in your enlarger and project the line. Now measure the line on the base board (lets say 8 cm).
3) Calculate the enlarging factor for this line: 1 cm/8 cm = 1/8 or (0.125).
4) Put the pinhole in your enlarger and measure the diameter of the projected circle (lets say 1.2 cm).
5) Multiply this factor with the measured diameter: 1/8 x 1.2 cm = 0.15 cm.
Now you know your pinhole diameter is 0.15 cm.
* it doesn't matter how high you turn your enlarger: but use the same hight for both measurements!
* it also works with inches ;-)