^ If you're not sure what your tolerance is but suspect it is "way down" then you want to be dosing as low as possible. I couldn't comment on how much of a bag to do - the heroin content (purity and also weight) varies hugely from place to place, so it's difficult to give you an accurate dosage suggestion, but if you are considering two options then definitely go for the lower dose. You can always wait for it to hit you then snort a bit more if you are sure it will be safe (make sure you wait long enough) but you can't subtract some if you take too much. As with snorting you consume the whole dose in one go, rather than over time (such as with chasing), you can't titrate it and the overdose risk is therefore higher.
Make sure you avoid any other CNS depressants too (eg benzos, alcohol) and take it with a friend rather than alone