Cramer said it's practically impossible to call a bottom in a stock, so if investors are looking to buy 400 shares of an $90 stock they should buy 100 shares right now at the $90 price. Then, if shares slip to $85 a share, buy another 100 shares. Buy 100 more if the price hits $81, then the rest if it falls below $80. Cramer said he prefers to use wide scales because it affords investors more flexibility. The worst-case scenario in this example would be the stock going higher than $90 a share, in which can you've just made money, albeit less than you were expecting. But in the long run, buying more as a stock goes lower is the right way to buy.