I need to finalise the location of the gantry. From what I can see, the 'best' location for the spindle centre line is the middle of the machine. Keep in mind that once the gantry is fixed, its fixed.

Any reason I would not have it in the middle of the machine? The table pretty much moves the full distance of the table, except around 30mm to the rear which is where the ballscrew bearing holder is.

I could play around with table sizes, considering having a tool rack if I go with BT30 and like the Datron machines (but they are moving gantry) have tools mounted on the rear of the table for the ATC. Most of the machine images I can see online seem to feature the spindle generally in the centre of the table length.