I don't worry about using any recommended ratios. I mix it differently depending on the use.
For making rock castings, I mix the plaster on the soupy side. I use fast-setting plaster of Paris for rock castings.
For creating a hardshell for mountains, etc., I use Structolite and mix it more on the stiff side. The stiffer it is, the more rock-like texture you get from the Structolite. I'll adjust mixes based on this fact and the type of landforms I'm working on.
IMO only those individuals doing critical casting of parts, such as for those excellent Hydrocal building kits, have to worry about ratios and maximum strength.
For the rest of us, the materials are very forgiving. 
Jim
