If what you're getting is anywhere close to decent half a gram is definitely excessive haha. I'd start with 100 - 200mg total, and just cut out 20 - 30mg lines and snort those every so often to get a feel for how potent it is and how it effects you. Even 200mg of decent quality meth will keep you going well over 24 hours.
Just a heads up, ISO seems to be doing the rounds in Adelaide again (as well as at least parts of Sydney). Was all the usual stuff going around last week, then out of nowhere it seems everyone has batches with at least some amount of ISO in them. One of the first I came across was so bad it made myself and a friend fairly sick, but even sources which usually have great stuff seem to have a bit of ISO mixed in with their gear. Keep an eye out and remember that it's not always immediately apparent when smoking, since the meth seems to come off the puddle before the ISO does.