the way I true all of mine is to let it dry out completely and find yourself a paving slab (I have one just for that purpose as I'm a mile from the nearest pavement!).
Rub the stone back and forth across the surface until flat
if it's taking too long or the slab is too smooth just sprinkle some fine dry sand on it
I did try the truing blocks but to be honest if you want a truly flat stone they don't work.
btw. I generally use the 250/1000 stones for teaching and they are what I recommend to people. I use the single grit bricks as I tend to get through them rather too quickly (last year I sharpened about 2000 knives and lots of other things such as chisels, planes, drawknives, etc on the waterstones). The only time I would use the 1000/6000 is for carving tools that need a fine polished cutting edge.