What is the rule of thumb for height of a outlet or switch.

Outlet are around 12" and plugs/switches around 48". I don't think there is actually a code for height but I could be wrong. Mine in my workshop are almost 50" off the floor.

The 48" height for the switches is not good if you are going to install a wainscotting. Most of them are four foot above the floor. Also be careful if you are going to have a chair rail, you need to determine that height.
Also watch where you put the plugs so many times they are in the center of the wall and later you will find they are almost unusable because a dresser, bed, sofa or some other piece of furniture is covering them up, or if you do use them you have to have an extension cord to reach your bedside lamp.

Okay,

The general rule is that receptacle outlets are 12" in above the "Finished Floor". Also, in general, switches are mounted at 46" (To center) from the Finished Floor.

A switch cannot be located at a transition point on the wall. For example, in your kitchen, if the 46" position lands you with 1/2 the box on drywall and the lower part of the box within the ceramic tile back splash, the switch box has to be completely in one or the other, not both.

95% or better I see 18 to center for recepticles(16" to the bottom, or 20 to the top with a 4" box.
And 48 to the center for switches?

I use 48" to the top of switch boxes and 16" to the top for general use receptacles.
Kitchen, bath and garage receptacles are also 48".