ok, heroin dissolves completely in water, what get's caught in the cotton you don't want in your rig most likely. I'm from new york, and the amount of dope that comes in one bag will ammount to about a one inch line I would say. I can disolve that in as little as 10 units, though I usually use 15-20. .5 rigs are good I think, I started out using these. Your best bet to make sure you don't miss, is to practice with sterile water. Or if you have a friend with you, fix each other up. I'm not sure what kind of needle you have, but the .5's I got, the barrells are the same circumferance as the 1 cc needles, but they're half as long. This makes it easier for me to work the plunger. Everyone's different though.
I remember the first time someone showed me how to prepare heroin properly in a syringe. I was shocked by how little water they used. It might look like you couldn't possibly be getting all the drug with that much, but you are, believe me.
I also wanted to ask you if the dope you're planning to inject is from the same batch that you've been snorting? If it is, then I would try shooting a little bit less, because you'll be getting 100% of the drug, and it's going to hit your brain immediately. For safety's sake, and for the sake of not trying to push your tolerance up too fast, you'll want to start out with a little less. Also, make sure you're sitting down, or on a bed or a couch. The first time this kid fixed me up in a rehab, I collapsed, and probably would have hurt myself had their not been a bed behind you. Also, if you don't feel the shot immediately, do not do another shot right away. This means you missed, but even a missed shot will get you high, and you don't want to overdose.
If your buying your dope today, or whenever you plan to do it, still test it out first by doing a very small amount. Just because it's from the same dealer, or comes in the same bag, doesn't mean it's the same product.