I've been spending much of my time over the last month trying to develop a short term tech trading system. I am reasonably happy with what i have so far but am struggling with buy prices.

how predictable are day lows/highs?
Currently my program is buying at a day low and selling on a day open. so i'm not too concerned with highs.

I'd like to get some ideas on how to more accurately pick a buy price, rather than using a straight % of the previous close for eg. What are the main influences on a day low?

any help would be great