Newbie design

[JonathanP]

Hi this is my first attempt at a CNC router, I am looking for faults in the design, as I have no experience any help would be appreciated
Parts to be used are as follows
Main screw SBC 40mm 20mm lead C5 accuracy with 5.1 belt reduction Max rpm400
Y Axis screw SBC 25mm 10mm lead C5 accuracy with 3.1 belt reduction Max rpm 700
Z Axis screw SBC 20mm 5mm lead C5 accuracy with 3.1 belt reduction

Guide rails
Main X axis 30mm SBC linear rail system
Other axis 20mm SBC linear rail system

Motors
Main X axis 1kw AC Sanyo servo
Other axis 400watt AC Allen Bradley servo

Drive system
Granite devices

Gantry is made from 100x200 t-slot

Questions
Is there a way to calculate how far apart the Main gantry bearings should be placed ,ie is it a ratio of the width of the gantry

Thanks