Since I used a different yarn weight and a smaller needle size than suggested in the pattern I worked more repeats to make the shawl as big as possible and use my yarn to full capacity. I reckoned that I had to start the double brioche section with a particular number of stitches and made up this formula:

Full number of stitches minus border stitches (10). The remaining stitches must be divisible by 4 :

brioche pattern (divisible by 4) - 1 st for starting yo - 2 sts for center spine - 1 st for final yo

Applying this calculation I increased 5 sts instead of 1.

According to my swatch I went one needle size up for the double brioche border. Regarding the finished shawl this would not have been necessary.

I inserted lifelines before I switched to the next section.