I agree with Crappiefarmer, add just enough water until you don't see any more dry concrete then add just a little bit more. I mix mine in a wheelbarrow then transfer it to my tubs or buckets.

I believe,from past experiences, that you can add too much water while mixing. I like it to be well mixed but a little hard to stir around, not soupy.

Another thing to watch out for is that you don't place your PVC or bamboo in a fashion that would make your concrete have weak areas in it. I have had a few PVC beds that I made in shallow dish pans break apart on me when I tried to move them.