Originally I was using a flat add to the raw Reflex Defense of the armor (i.e. adding two to the value to get the DR). After a bit, I decided that I wanted stromtrooper armor to have DR 10 (enough to pretty much stop a sporting blaster from a low-level punk, but not enough to stop any serious firepower), so I rearranged things, using that number as my baseline and just counting up or down from there.
By applying mook rules to stormtroopers (any damage that gets through pops them), this allows me to grant some pretty significant protection to important characters while preserving the ability of the pcs to mow down dozens of troopies.
I applied the same thing to vehicles, adding a value to the vehicle's armor bonus and adding it to the existing DR. I think I added four, there. Also, mook rules apply to TIE Fighters.
Another thing that I did that relates to this was that I made Ion damage bypass DR, since it isn't really meant to damage a device so much as to disable it. This has made ion weapons somewhat more effective.