1) Take a piece of clear film and make a mark of 1 cm on it with a pen like this |----|.

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.

Note:

* it doesn't matter how high you turn your enlarger: but use the same hight for both measurements!

* it also works with inches ;-)