I've done .25 gram in 40 units of water fine. All tweakers say use the least amount of water possible, I tried searching for meth-amp's exact solubility in water figure's for you without luck, sorry.
I'm not exactly a tweaker, or rather, I dont frequently use methamphetamine. Other stimulants yes. I have IV'ed it a handful of times though. And I have truckloads of experience IV'ing other stimulants. Anyways, what I'm getting at, is:
Coming from a safer injecting / harm reduction stand point, I would try to use a healthy, generous amount of water or saline per shot, and not "use the least amount of water possible". The reason being the more you dilute the solution before injecting it into your veins, the less damage it does while traveling through your veins. I believe methamphetamine and stimulants in general are often very disagreeable with the side walls of your veins, so IMO it would be in your best interest to dilute the shot as much as you are comfortable with. You of course do not want to miss a shot of methamp, so when you are choosing your amount of water, go for a compromise between as much water as you can fit and keeping the syringe of comfortable length.
Again, there might be a reason specific to methamphetamine why people have said "use the least amount of water possible" (maybe its to keep the missed shots at a minimum?), but as far as I know this is contrary to the general harm reduction opinion. Which is the most important opinion, when dealing with IV use.
For most of my injections, I use between .5cc and .8cc water. I shoot with 1.0cc syringes.