I found this interesting tool that calculates today's worth of your hypothetical investments in the past. Except I'd call it "How much I regret for not investing" calculator: http://newinvestordaily.com/what-would-it-be-worth/
At any reasonable interest rate, i, in percent, the time, T, in years for an investment at that constant rate to double is T = 72 / i. With only a couple of percent error.
The value of estimating speculative returns given perfect hindsight is valueless unless principles emerge which allow one better than average investing the future. No one has a time machine to realize the perfect dance about market trends any more than one can go back to bet on yesterday's sports results. Picking individual stocks or attempting to time the market are strategies that pit the individual investors with emotional hunches against chummy executives who control the flow of financial information. It's a losing bet. The value of estimating the range of likely future returns given average investing returns is a measure of reliable long term plans. It's not as sexy as speculation, but while attempts to beat the market is inherently risky, one can very closely adhere to the average of the market of one's choice.
