I usually try to aim for pirating about 40% of what a trader would make on a run, which in my trading experience is a bit more than 1000 per unit for a shortish inter-house journey. This usually equates to 100-200k for a freighter, up to the full 2 million for a 5k transport. If you're carrying certain types of cargo, then my pirate will let you off for a small amount of the cargo (usually 20 units maximum). If people lolpirate you, I find that the best way to deal with it is to pay them (or die, your choice) then tip off the house lawfuls/bounty hunters/mercenaries and wait for the little blue (or red) message saying that they've been put out of action.
