So, when designing - be it with colours, or with texture - you need to think about how deep to make the border of the shawl, and hence, how much yarn to reserve for this. I've just spent 20 minutes thinking about this, and have come up with a handy chart for rough-and-ready reference. The "rough" part is that this chart works on surface area. It assumes that the stitch pattern (hence the density of yarn) is the same for the border as for the rest of the shawl, and that you're making the border of the same type/weight of yarn. If you are making a lacey border, the stitches may be less dense (ie. less yarn used for same area) - swatching can help you adjust the numbers in my chart.
Here's a picture of your shawl.
Let's say that the main body of the shawl - orange in the picture - has depth 1 unit (that means the distance from the neck edge to the point), and you want a border - the blue bit in the picture - running somewhere from 0.2 units to 1 unit. The former corresponds to a narrow border - example: 12 inches of orange, with a 0.2x12=2.4 inch deep band of blue - while the latter corresponds to the case where the border and the body of the kerchief have the same depth. The chart below then gives you the fraction of yarn you need to reserve for the border, relative to the amount used for the main body.
|depth of band||fraction to reserve|
For those who want to take this further, the formula for generating the right hand column (y) rom the left hand column (x) is: y = 2x+x*x. Or you can derive it yourself from basic triangle geometry!