I have yet to figure out a perfectly thick frosting recipe that keeps its shape when piping. (I use round tips mostly).

What is the variable ingredient to add/remove to get each of the different thicknesses?

Can anyone compare frosting recipes containing each of the following consistencies?:
-Thin (poured icing)
-Medium (crumb coat)
-Thick (guaranteed to keep its shape when piping)

liquid (water) makes it thinner and powdered sugar makes it thicker