@nihilism - yes lots of people say don't use any more than 10% of your investing portfolio for a trade - however I would then ask that if you are young with limited capital to start with how can you possibly make this work. If all you have is say $5000 to trade with then using $500 at a time really doesn't work - you need a stock to change by a minimum of 10% before you've even covered your brokerage!
