According to Rudy :
available at the local big-box hardware stores. Unfortunately, the
packaging only lists the mixing proportions for mixing the entire bag
at once! Needless to say, I don't need 18 pounds of compound to finish
this small of an area.

Buy premixed, ready to use: It comes in smaller plastic tubs than the 18#
box, right down to about 1#

Here, HD has CGC 90 in what I think are 2 pound boxes.

I _think_ it said 3 parts compound to 1 part water, but I'm
not sure.

I've been so used to the readymix, I've not tried the setting
type before. But I just bought one of the aforesaid small boxes
to try it out.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.