You said "this should be technical", so what you provided would suffice for most audiences.
If this is the only part of the text that is utilizing the equation is the one sentence, then doing it as a parenthetical as you did is fine.
You could restructure the sentence to remove the parenthetical:
If you are fixing a 10 days port call in a month, having a 0.5 ratio would result in 5 days of rain stoppages because 10 days of port call multiplied by 0.5 is 5 days.
If your audience is assumed to know how the equation works already, you could omit the explicit math and streamline the sentence:
If you are fixing a 10 days port call, you would have 5 days of rain stoppages with a 0.5 ratio.
It isn't applicable here, but if you ever use an equation that stands on its own (like Pythagorean Theorem), you'd want to add it as a separate figure and refer back to it when you use it.
(Note: You need a comma after the "if" clause since it is at the start of the sentence.)