There are many ways to figure out scallops. I start out with CDs (remember when AOL came on free disks?? those were great), then move up through my various plates and dishes. I also have several large drafting compasses.
In this case, I agree with Quiltingshorttimer that a half circle arc would be too much and too harsh. I think if you based the circle going from the center of one set of squares through the point and to the outer edge of the next set of squares, that might be a nicer arc. I'd probably position the arcs midway through the blocks, to allow for that corner. Corners are a sticking point on curved edges, many ways to deal with them and sometimes it is easier to come up with your corner first and then make the rest of the border work with that.