Jonathan,
I have recieved an email from Gary and he descirbes the App/unit as being made up of mm per step of the motor/ seconds squared.
So it looks like you are correct in what you were saying. I will increase the setting to 500 first of all and see how the machine performs and maybe get higher than this depending on how it is. I have noticed in the newer version of the manual that the setting page does have a value of 950 in the Acc. box so I will see what I can get my machine to run comfortably at.

regards

Ian