I will share my experiences which will help to understand my strategy a little better. Before this, one thing is for sure. No one can predict the movement of markets (irrespective of how many channels you watch/papers you read/websites you read). So one shouldn't aim to try and time the market.
Coming to my experience. Take one fund for example. Say HDFC Top 200. I have a total investment of Rs. 5000 p.m. as the SIP per month. This I had structured as 5 SIPs of Rs. 1000 on 1st, 7th, 14th, 21st, 28th of every month.
Being a devotee of mathematics and statistics, I started observing the movement of markets and their impact on my SIP. To my view, I started feeling that though there is a lot of volatility in the system, during my SIP days, more often than not, there was an upside movement. This means that due to the NAV being calculated on the same day, I would get lesser units. I observed this pattern a lot of times over a year and half.
Coming to additional purchase. If I need to average out my investments so that my cost value is low, then I should purchase more units at lower price. Say in a month, I purchased Rs. 5000 worth of units @ Rs. 200 per unit, I get a total of 25 units. However, if I need to get it down to say Rs. 192 (4% lower), then I should purchase 100 units at an average price of Rs. 190, which means Rs. 19000 extra investment. This is a huge sum and definitely not possible to implement over 6 funds
Your point of 10-15% is valid. However, there are couple of factors we need to think about. One being the timing part which I quoted earlier. The other being the frequency of 10-15% correction. It will not be very often and hence, you will have a very limited window as any such correction will mean that there is a knee-jerk reaction and very quickly the markets will be stabilized. Having said that this is definitely an opportunity to pump in some spare funds if one has.
I hope I am able to provide a little more clarification on my strategy.
Happy Investing !!