I prefer not to use spacers at all myself to increase the difficulty. We don't break for competition so it makes sense to save a bit of money by asking a student to break 3 unspaced boards instead of say 4 spaced boards.
Do you feel it's safer to use spacers? Just curious why you choose to use them.
Most of the competitive breakers I know use something like a quarter of a centimeter. I don't believe there's a real science to the number - it's based on whatever they scavenged from Home Depot to do the job.
Spacing is cheating good for flash and trash demos. The spacing would vary by "brick" fpr optimal energy transfer. Just enough so brick has passed it's elasticity limit resulting in a break, but not so much that any energy is lost.