How much weft for a towel?

Question: I’m looking at designing a tea towel but cannot find a reference for calculating weft requirements. The organic cotton I purchased has I believe 710 yards per cone. Will this be enough for a towel or should I get another cone of each colour before beginning?

One cone should be enough 18 ppi x 20″ wide = 360 ends x 30″ long = 10,800″ divided by 36 = 300 yds of weft per towel approx.