Even when its pure, crystal is hard to guage. Set and setting can play a big role, as with all drugs. If you want to be buzzing for 8 hours, I'd say just take it real slow at first and pace yourself. Dont try to go for one big hit that will last for ages, because you'll just end up making yourself sick, and the comedown can be murder.
If you snort it, it wont be as euphoric, but in my opinion, it lasts a bit longer.
Its very hard to get precison when your snorting or smoking drugs because you waste so much. I think two points would be enough for you if you pace yourself, but I'm not hugely expeienced with crystal as I've only tried it half a dozen times. And never more then half a point smoked through the evening (I'm incredibley lightweight).
*EDIT* Sorry if that didnt make sense; I'm all over the shop this afternoon.