EstlCAM + Arduino + D420 = scale issue on steppers???

Hi Everyone,

I've just setup my D420. Great looking piece of kit. I opted for EstlCAM as a controller using the Arduino UNO, connecting to the Stepcraft parallel port.
I have home'd the machine successfully. The limit switches are doing their job. I had to invert the Y axis travel in the software setup... not sure if that is normal or not.

The Problem:
When I create a simple tool path in estleCAM and run the Gcode, the steppers move way more than is indicated on the EstlCAM on-screen preview. I have to hit the Estop before the gantry hits the edge... but my tool path is supposed to be in the middle of the bed.

Here are my settings that I flashed to the Arduino:
Steps per resolution 1600
Distance per revolution 5mm
Max feedrate 3000mm/min

This is likely a really basic mistake. I'm pretty new to this so any help is much appreciated.