The pirates OP mentions were actually sanctioned for other offences. They were indeed not the best example on how to roleplay a piracy situation. I guess you should let yourself get pirated by someone else. (Hint: callsigns in all-capital letters are probably not what you are looking for).
As for appropriate demands - I think the top is roughly the maximum value of the commodities you're carrying, plus minus a million for larger and hundreds of thousands for smaller ships. Just a basic rule of thumb, there's a ton of exceptions.
