Question 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.

Usually steppers have 200 steps per revolution and the stepper driver runs the steppers in half stepping mode on the D series.

400 steps per revolution and 3mm per revolution are the real parameters.