There is no exactly value. So the base is just "roughly there". However, you can read the history of the chart to see how past base get respected. If the bounces always go past base, it's a high probability that this time it would do so as well, so you can set your sell at ~ candle body. If the bounces often return with a slight touch or with a wig, better be safe and sell lower than base. The answers are all in the chart