Make custom col in ionicframework

These are the existing cols

Explicit Column Percentage Classnames
.col-10 10%
.col-20 20%
.col-25 25%
.col-33 33.3333%
.col-50 50%
.col-67 66.6666%
.col-75 75%
.col-80 80%
.col-90 90%

How do you properly create a custom .col like .col-15?

UPDATE

in CSS I am now using something like this

.col-15 {
    flex: 0 0 15%;
}

Let me know if theres a better way than this. Thanks!