The general concept is to have your primary monster (in this case shockers), be the default monster, and assign the other monsters appropriately to get the ratio you want.  The default gets however many chances are left over after the rest out of 1000.
Probably something like set electric to default, and all the 20's to 100 or so, that would give you about 40% shockers and 60% everything else, but the everything else would be mostly split 6 ways.  Adjust as desired for your ratio.
I THINK the result you'll get with this setup is that you'll only get about 22% of the spawns that the map calls for, because that's as much as is accounted for, and it'll spawn "null" monster otherwise.