I read that a scale is say 1:64 which means that the model will be so many mm long. How is this worked out? Is there a formula to work to.

Example :-

The average height of a man is lets say, 6 feet tall or 72 inches.

If you divide this figure by the scale, in your case 64 then the result is the size of a figure for you scale i.e. 1.125 inches or to round it off, just over 1 inch tall.

If you have a model figure of which you dont know the scale, then take the full height (in this case 72 inches) and divide that by the size of the model figure (1.125 inches) and you'll get the correct scale (1:64).

It gets a little more complicated when converting from one scale to another but it can be done with a bit more calculation.

Terry. :)