Ok. Z AXIS design time. What is the acceptable leverage ratio of a sturdy Z?

My length of linear rail is 450mm. I was hoping for 300mm
Of cutting depth but it seems like there would be excessive difficulty keeping my Z cutting straight with a 150mm (1/3 ratio) binding area for the spindle on a 450mm run. What do you all suggest?

Also, would be wiser to mount the linesr bearings on the spindle plate or on the gantry plate?