Since you are choosing the offset z-axis option you should know that you will need to modify the way the bed is mounted to allow for clearance between the lead screws and the bed. The stock options will likely result in the lead screws hitting the bottom of the bed. This means you will essentialy invert the bed mounting (bed will be mounted inside the extrusion rather than outside). This will probably require an additional 20-40mm of length to accommodate vs stock. I recommend adding 30mm to the bed width in the calculator; but you should double check or ask about this before finalizing cuts.