First, I found calb1’s notes really helpful. If you’re interested read here:
http://www.ravelry.com/projects/calb1/eisig-warm

Meanwhile, to get the correct width with my yarn, I cast on 28 (for 18 cm which will be about 24cm when the border stripes are added).
Since it’s in 6 sections (3 green and white / 3 grey-stripe-grey) I divided up the full measurement by 6 and then estimated based on my swatch the size of each section - so again, reduced to 28 ‘ridges’ so that the whole thing doesn’t end up trailing on the ground!

still haven’t put two panels together so I’ll let you know if calb1’s technique works for me :).

Now that I have tried various methods (sigh) I can say that using a garter grafting stitch and treating the white and green striped panel as the ‘pearl’ row and the light grey as the ‘knit’ row it came out looking acceptable. I tried doing the three needle bind off version but the join came out with the stitches looking too ‘slanted’. The edge where the stitches are picked up looks great!