Sell enough of a winner to take your original cost off the table, and whatever is left rides with nothing of yours in it. This computes exactly how much that is — and what the remainder is worth in each outcome, including the one where you were right and sold anyway.
commission plus slippage
Target
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Sell now
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Cash back
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Riding free
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Worst case from here
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if the remainder goes to zero
If price goes to
Hold everything
This plan
Difference
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What this does and does not do. Taking cost off the table does not raise your expected return — mathematically it lowers it whenever the position keeps rising, and the table above shows you exactly how much that costs in each outcome. What it buys is the ability to keep holding: a position with none of your original money in it survives a drawdown you would otherwise be shaken out of, and it frees the capital to be somewhere else. That is a trade of expectancy for staying power, and it is worth making only if you would otherwise sell the whole thing at the first scare.