Most of us don't the math, because it takes longer than the few seconds that we have to make a decision.
There is a commonly used simple shortcut. Use the outs to calculate an approximate percentage. A gut-shot has 4 outs, a OESD straight draw 8 outs, a flush draw 9 outs, an straight draw with an A has 4 outs. If you have an A hole card, then 3 more outs. Just examples.
Take the number of outs times 2 and that is the approximate percentage that the next card will help. Multiply by 4 for the next 2 cards to come. 8 x 2 = 16% for the next card. 4 x 4 = 16% for the next 2 cards.
Just a simple trick, close approximation, but handy.
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