How much citric?

Most of the heroin illicitly produced in the world today is the "number three" base form of the drug, which is usually brown in colour, but can be grey or white. This base form of heroin is high purity at point of manufacture (though nearly always adulterated by dealers) yet readily smokeable. (The heroin is put on a strip of tinfoil and gently heated from underneath, which causes it to melt into a little puddle. The foil is tilted and encouraged to run and the fumes are inhaled through a foil tube stuck in the mouth. This gives a better "bioavailability" than snorting (ie more of the drug taken actually reaches the brain), but to smoke anything more than a tiny dose this way takes tens of minutes as the heroin only vapourizes fairly slowly... Many long term users prefer to inject heroin as all the drug taken hits the brain in a matter of seconds, saving money and maximizing the high.
Injection-quality heroin hydrochloride is produced in small amounts in Afghanistan and, more famously in the Golden Triangle area centring on Burma, where the product is known as China white, because production and distribution has long been controlled by ethic Chinese gangs from Thailand, Vietnam and surrounding countries. But "number four" heroin as it's known in the trade, is more expensive than the more common "number three", and is considered by dealers to be a less versatile product, because it cannot readily be smoked. Injecting addicts, however, usually prefer number four heroin because it dissolves easily into plain tap water without the need to add citric acid or vitamin C, which can sting and cause damage to veins.
This video uses actual heroin of about 75% purity to demonstrate the "cooking up" process and shows how much citric acid should be added and what the resulting heroin solution should look like...
Too much citric damages veins!