Krista just posted quick tutorial on scalloped borders to her website blog-
https://www.kristamoser.com/blog. She has templates you can print and use to layout the border and so simple to do without all the math. She releases a technique/tip/guidance every Sunday and posts them to her blog so they can be revisited anytime. She also has link to the pattern shown Many Morsels Scrap quilt which she is offering for free. Love Krista Moser.