My buy range is when price falls below fair value. Its that simple. the difficult part is finding fair value. My valuation model is described here. I use it extensively to get an estimate of fair value. Calculation depends on how well one understands the business so everyone will have a different notion of fair value.
Fair value is dynamic. It depends on earnings, risk premium, interest rates, growth rates, news flow etc. There is always something that will change every day. Some events will have larger impact than others. On an average, I think these stocks are still overvalued so a 20% drop from here will be a good low-risk entry point. It does not mean I expect these stocks to drop 20% from here. It only means if a stock 20% from these levels, risk of loss of capital will be low buying at that price. A stock gets into this list if I am willing (and able) to buy more of a stock if it drops 20% due to valuation reason. If a stock will appear overvalued even after a 20% drop then its too overvalued to be shortlisted.
If you have track shrimp prices, who’s selling stake, custom duties, production in China etc (factors that companies themselves cannot control), then it means stocks are priced to perfection and investors are on the edge of their seat. Investing in such a situation mean expected return will be average (approximately 10-12% for a basket of stocks). Investors weren’t worried about these factors when stocks were selling at single digit PE ratios few years ago.
IMO, buying right company is more important that paying the right price as long as you are not paying an exorbitantly high price. I think its not too difficult to tell if the price is too high. you don’t need a complex valuation model for that.