AutoCAD 2013 :: Creating Size Of Object

I am creating flat patterns of sheet metal parts and i would like cad to do some calculations for me.

For example when i am creating an object , be it a rectangle or simply a line , i dont want to have to work out my sizes on the calculator then input the size to create the object. I would like cad to do the sum for me by inputting my sum straight into the dimension of the object i am creating. E.g CLick rectangle, select a start point and type 1200-375+37+37+10+10, Then tabbing to do a similar thing for the width

The way i draw flat patterns is to start with the overall size then chop into that shape to create a finished pattern. I also have used Inventor and a software for cnc punching called Cadman. Both of these programs you can do this and what with Inventor also being a creation of autodesk i would have thought this could be done in cad as well. 

Is there an add on or a simple command that will let me do this.