I would to know if there is a mathamatical formula for 1/72nd scale stuff. example: lets say you want to build a diorama that has 1-2 signs in it and now that you know that the real signs are approximately ah lets say 18in x 18in. Now I would like to know what size and thickness a piece of evergreen sheet styrene is needed to make these signs true to scale. I hope I made myself more understandable on what I am trying to do and need.

It's easy

You have just to divide by 72 all the actual dimensions

So, in your exemple

18in X 18in = 18 / 72 in X 18 / 72 in = 0,25 in X 0,25 in

Hey that was easy. I feel like such an idiot. anyway thats my old friend ziggy.

Jody;

As to styrene sheet for those little signs... IF the signs were on metal, use the thinnest Evergreen sheet (I think it's 0.005 inch). IF you are going to paint your signs, and you are comfy with metal sheet bits, using some PE scraps from some set which came on ca 0.003 stock would get you better scale thickness (thin-ness)... a real metal sign might be painted on 1/8 inch (or less) sheet metal - which would be 0.125 inch / 72, or less than 0.002. IF you are doing a wood sign, of course that could be thicker by far... half-inch plywood for example would be around 0.500/72 or ca 0.007 inch ... you could use either 0.005 or 0.010 styrene sheet to approximate this.

IF you are trying to print a scale sign on your typical printer; most copier paper is around 0.005 inch thick - so a printed sign could be cut out of the paper and stuck on a post and be fairly close to scale for something between 1/4 and 1/2 inch thick. Photo paper is thicker and makes a better "board" sign.

Hope this helps!

Bob