Here is a handy gadget from the Fimo people - a flexible ruler and proportioning template.
This template shows measurements in both metric and imperial. It also has a proportioning guide for the segments of a small block of Fimo. The other really useful thing is a range of holes in different diameters - allowing you to easily measure the diameter of clay balls and logs and also judge how many segments of fimo each ball equates to.
If you do a lot of colour mixing, this little template is bound to be very handy indeed.