OK, it's an easy question, here we go. Say you have an object A in 1:X scale and you want to do it in 1:Y scale. You take a measurement from the model, multiply it X times and you get the real (1:1) measurement. Then you divide it by Y, to get 1:Y scale measurement. You could make it all easier for you if you first calculated X:Y ratio. Then if you take the 1:X scale measurement and multiply this by X:Y, you get straight the 1:Y scale measurement. To take your case as an example - you have 1:18 scale object and want to know it's measurement in 1:12. 18:12=1.5 so you multiply your measurements by 1.5. 3/4'' X 1'' in 1/18 would thus be 1 1/2'' X 1 1/8'' in 1/12. It's a lot easier when you go metric - 1 inch is 25,4mm. Hope it helps, have a nice day
Paweł