I pluged in the your shaft to efunda.com
If I did it right (before my free trial membership ran out)
I used a center center load of 40lbs (18.1kg) and it calculated a deflection of 0.033mm at the center.
I used that to get my shaft deflections when I built my machine.
Forgot to add:
if you use 2 shafts, you will get about half that deflection for the same weight. plus your probably supporting the ends better then a simply supported beam, so deflection will be better.