How do you tune ports when making a sub box?

I am building a ported enclosure and I don't know how to tune the ports. I want to have slotted (rectangular) ports. Does anyone know how to calculate the length and width of the port to tune it to a sub?How do you tune ports when making a sub box?
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http://www.the12volt.com/info/diagrams.a鈥?/a>How do you tune ports when making a sub box?
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