Just to clear things up a bit. For reduction ratios using toothed belts, you can calculate using tooth counts. Where I was seeing an error was the actual belt that is the X axis. Basically my X axis is a continuous belt of about 2.5 metres (5mm pitch). This belt rides on a 26 tooth pulley at each end of the axis. Now if you rotate the 26T pulley one revolution, the belt should have moved 26 teeth. Which at 5mm pitch is: 130mm. This is not the case though. As you can see, if the belt were thicker the outer diameter around the pulley would be larger and the belt would have travelled further.
I think.....