Dixie - I hope you are doing some research before trying IV coke. From your posts above it doesn't sound like you have an addictive personality; even your reference to trying crack.
Before I answer your two questions above, be forewarned that there are three primary risks to shooting coke that outweigh the others. Firstly, for me it far more psychologically addictive than any other substance/ROA. You might like it so much that you end up addicted, even if, like me, you could take or leave pretty much anything else.
Secondly, quality varies so greatly, not to mention tolerance and body weight. If you follow Molly's suggestion above, with good quality blow a .2 shot will buy you a trip to the emergency room (if your lucky) and a trip to the morgue if not.
Lastly, because the intense cravings will make you want to keep shooting and shooting until all the blow is gone you will be prone to acting more recklessly as the session goes on. For this reason, it is a good idea not to have more on hand than you plan to use, and go somewhere where you can't buy more. Your judgement will approach -0- a few hours into a good session. Don't let decisions you make then cause you trouble in the future.
If you are starting with an 8 ball, you can pre-mix it with 10 ml of saline or sterile water, then filter into a sterile container ( l liked to use an empty Visene or other eye drop bottle.) I weigh a little over 200 lbs., and I would always do a 50 unit test shot to determine the purity. Assuming you weigh less than me, you can figure out what dose that should be for you.
10 ml saline to 3.5 g cocaine is a good ratio for beginners. If you miss a shot, it is not so strong that it will kill the surronding skin tissue causing a skin lesion. At that concentration a 50 unit shot will contain a maximum of about 125 mg of cocaine. Assuming you have good coke, (30-40% + pure) a 100 - 150 mg shot should give you the experience you crave.
Another suggestion would be to pre-load a few shots before you get started. I would preload 50, 60, 70 and 75 unit shots. If each preceding shot went OK, I would do a progessively fatter shot each time - up to 75 units. Preloads will keep you from trying to load shots when you have the shakes and dull the needle. Hitting each other should help as well.
If you use a tie-off, make sure you release it before you push off. Then push half, count to ten and push the rest if you feel OK.
Be safe. Have fun
FC