How to border a circle a FSL circle help please?

Hi all, I am dabbling with free standing lace at the moment and have managed to successfully make a free standing lace circle that will hold lettering without falling apart, I need to stop a bit of pull in places but nothing major.

I am however, struggling with the border of the circle. The circle itself is 87mm in circumference and I have applied a border in a zigzag stitch of 2mm width and a density of 5.0 line/mm. All stitches out well and holds together, but when I have soaked it in water to get rid of the solvy, that border 'wrinkles' up a bit so the circle doesn't lay flat at the edges. Do I need a thicker border or a different type of stitch for the border do you think?

Many thanks in advance for any input :)

Many thanks for your reply AlisonB! I'll have another play at the weekend...

Another question if you or anyone can help please - When it comes to resizing the circle (Smaller) I hold down ctrl and drag to maintain the proportions, stitch count and density, however I think this is likely going to give me problems on stitch out, so is there a 'golden rule' to follow on reducing stitch count and density after resizing or is it all trial and error? say if I were to reduce the size of the circle by 50% do I need to reduce the count and density by 50%?

I have a feeling it's not that simple :D

many thanks again :)