we can't tell you how much to use without knowing how pure your product is. I would start with 50mg at the most (25mg would probably be smarter), mixed with water at room temperature, stirred with the clean tip of the plunger (immediately after breaking the seal of the cap), then filtered with preferably a sterile filter. 50mg most likely won't be enough to get you the rush you are looking for, but it is a good place to start. when I I.V.d I would do between 100-150 mg of the strongest coke I've ever found, and with cheaper coke the the dose would go up to 500mg at times.
shooting coke is one of the least satisfying drugs I've ever done, yet I did it on a daily basis for months. I sold everything worth value I owned, chasing that first rush. You need to be very cautious with this one, I would always try to get a "bellringer" which is when you do a high dose I.V.'d and sounds start flanging (usually the sound of the sink or toilet since I usually did it in public restrooms) and there is a distinct high frequency ring. the problem is the dose needed to achieve this effect is extremely hard on the heart, and only milligrams away from the dose needed to die. I.V. coke is (in my opinion) the most dangerous way to do the drug, and (also my opinion) the most addictive drug. The rush only lasts about 5 minutes, and after that all you feel is the regular effects of cocaine, but you won't enjoy those effects any more.
You also need to be very careful when injecting, due to the numbing nature of cocaine you could inject an entire shot into your tissue and not feel a thing. because of how much cut there inevitably is in cocaine, and the varying chemicals used to cut it, the risk of abscesses is extremely high. cocaine is also a vasoconstricter so the more you do the smaller your veins get. I've gone through grams I.V.'d and only succeeded in getting one shot all the way in the vein. not to mention your hands will often be shaking, and the urge to do more will be very powerful, and often outweighs the urge to use hygenic injecting practices. with shooting coke needles get shared very often. I'm not saying all this will happen to you, I'm just informing you of the risk that you are taking. If I could go back and change the past regarding my drug use, the one thing I would certainly change would be to never get into I.V. coke. it is far more trouble then it's worth.
if you have any specific questions just hit me up. I'm very versed in the subject, and although I've made poor choices regarding it, hopefully you can learn from my mistakes.