I think what
@limster is saying, is you need to get the market returns percentage over the period right. For example, your investment horizon is 10 years and you are investing in S&P500. In the next 10 years if lets say S&P 500 returns double, i.e 100% returns. But with lot of volatility in between. If you know before hand that it will be 100% in 10 years, then you can draw the value path and then start investing and you will make awesome returns, because you can use volatility and buy low sell high and make more than 100% returns.
But if you underestimate to 50% returns, then as the market rises, you will keep on selling your stock or allocate lesser and lesser and in the end, you end up with lot of cash.
Similarly, lets say if S&P 500 gives only 20% total return over 10 years but you are expecting 100%, then you think market is very cheap and you keep deploying way too much cash and finally run out of cash in the middle.
So getting that value path over the long term, correct is important.
But I see the value in this value averaging, because I am assuming 0 returns and plan to step up my allocation ratio, so I think this works well for people who are stuck with a huge lump sum of cash and want to deploy it into the markets in a moderate risky way. Lump sum would be too risky, DCA is fixed amount, so you dont make use of any corrections to buy more.