Hi Marvin,
We exit a position when the reason that we hold the stock has gone, especially from a risk / reward perspective, & / or the stock no longer represents the value it once did.
It’s our belief (derived from experience) that setting an arbitrary level to exit, such as 10% down in your example, is not aligned with our top-down meets bottom-up approach. Putting in place a ‘stop loss’ like this should be used in mechanical trading systems that reply totally on price. Whether that be momentum systems, regression or one of the many technical trading strategies out there, but when a strategy relies on a combination of fundamental & technical analysis i.e. not as mechanical in nature, then arbitrary levels are counter productive over time, in our opinion.