I've made myself 5 rules for playing with coke:
1) only ever do measured amounts
2) set a limit to how much you allow yourself to do at one time
3) set a time limit for when you allow yourself to take a subsequent bump
4) if you know you're going to want do to some at a specific time (party, event, etc.) completely abstain until that time
5) if you're not feeling enough effects abiding to these rules, its time to set it aside for a day or more
I stick to these rules almost religiously, because I know if I've done some, I probably cannot be trusted at that moment to make rational decisions about it.
I prepare a fair amount ahead of time and put it in the bullet. I then use what the bullet dispenses as my measuring stick for how much I'm doing. Even if I'm making lines, I dispense it from the bullet so I have a somewhat measured amount. For me and the bullet I use, two cupfuls from it make a healthy, average line. I allow myself to go as high as three per line if its the first hit, but no more. If its new product, I do only one cupful to see how it works for me.
For the time limit, I choose a minimum of 45 minutes. If I didn't note the time when I last hit it, I have to start my timing then for 45 minutes... no guessing is allowed because judgement about this is always going to be skewed in the wrong direction.
For an event, party, etc., I know that I'll want to be feeling the affects of it during that moment. If I've been playing with it ahead of time, I might be nearing tolerance levels at that point and I'll just want to break rules and do too much. So for specific events, abstaining beforehand is always the best policy.
If I am not feeling it enough within the limits of the rules, its time to set it aside and let myself recover as much as possible as I'm just wasting it chasing after something my tolerance at that moment won't allow. Any amount I do once I've reached this point is just being wasteful because "just one more bump" will not have any affect I'm looking for.