Hi Jazz and thanks for your response .
I dont think however you have quite clarified things for me.
So ignoring backoff for now .
I home to machine zero in all axes and the machine defines its position as 0,0,0
I have set soft limits X of 0 and 400.
Soft limits Y of 0 and 200,
and Soft limits Z of 0 and -50
So I have now defined a box 400x 200 x50 which is the envelope within which all work MUST be done .

We now come to the workpiece which I have to plant on the table completely within that envelope and clamp it. I have to bear in mind if I am doing an outside profile that I must consider the cutter and allow at least a cutter radius all round.

I am guessing that my workpiece origin or start point is defined somewhere within it , whether it is right hand end, middle, or wherever so I identify that origin, jog to it , and set all my work parameters to zero at that point.

So really it is only if I have placed my job wrongly and that the toolpath will stray outside the envelope that a soft limits alarm should arise .
Are you saying that it should arise prior to hitting the start button . or will it only arise when the work zero position is defined .