I think this is a good question too, despite all the wise guy answers from the "experts". Maybe they do not really know a good answer so they use sarcasm to make themselves feel better.
This is what I did. I measured the 50lb bag before I opened it. This gets me the rough approximation of volume (cubic inches) per 50lb bag. I then converted cubic inches to cups. 1 [US] Cup = 14.4375 Cubic Inches(http://www.asknumbers.com/CubicInchToCup.aspx#.UGLmfa7y_HU)
I then converted quarts to cups. (4 cups to 1 quart.) Reading the bag will tell you the number of quarts of water to add per bag so you can ratio down from that.
Using this method got me the consistency described as creamy peanut butter. I mixed a very small amount at a time because I was tiling a back splash and did not need much.
I wish the bag would give a table of mix using volume since it is easier to measure volume than weight. I cannot imagine trying to bake something where every ingredient was given in weight.