The blood bags have a numerical scale on them so it's impossible to screw up how much blood you are taking. Most all of them are 500ml bags, which is how much you want to take at once. Your veins in your arms that you would use for phlebotomy are/or should be very visible to the naked eye, unless you're a slob, and are nowhere near any major arteries, as arteries are much deeper than veins, so you would have to be an absolute fool to hit an artery. If you really want to know if you need to dump blood buy an at home hemoglobin monitor then you will know for sure with the prick of a finger if you need to do it. This really is not complicated at all and people are making it way more complicated than it really is.