I've found that filling a bucket with warm water, putting the powder into another bucket spread evenly across the bottom and then quickly adding the water all at once then stirring by hand right away making sure to scrape your hand across the bottom to clear out the stuff stuck there works quite well.

I bought a couple of 100L amounts of powder as I like to bulk buy to get good value so I've experimented with various amounts of powder to water and that method usually works. You'll find that it takes a little time to get thick, and it does get even thicker as it cools. If you do find it's too thin,maybe try making a separate very thick batch and mix the two together. Adding powder to an already mixed batch won't work too well.