Z axis question

Just wanted to know from the experts what the minimum distance apart you would fit the Z axis rails.
Looking to rebuild the z axis on my gerber router & the problem is that the gantry rails run along the front of the machine. Moving them is not really an option as there isn't enough room between the bottom of the gantry & the machine bed for them to sit top & bottom like a lot of the designs you see on here.
Trying not to make the z axis to wide
as I don't want to reduce the maximum cutting width of the machine to much.