Learning to Design Lace

I am a big fan of Evelyn Clark's designs and am playing with trying to learn to design my own shawl. Does anyone have ideas on books to look at or websites? Or maybe some thoughts? I'm especially stuck on how to build up the edges on either side of the pattern repeat, so that when you start the next repeat, you have enough stitches/

I've been working on the Thistle Lace pattern from Barbara Walker's Teasury, volume 2. My goal is a triangular shawl with a symmetrical, small repeat, then a border, then a peaked edging. The Thistle leaf pattern is a 28-row repeat with purling on all even rows. Here's where I'm stuck:

1. does the border edge have to keep increasing in number of stitches, or can I increase to the number that I need for the repeat (11) and then do plain knit or repeat the same row on either edge until I get to the end of the 28 rows needed for the pattern repeat?
2. any ideas on how the border should mirror the pattern? can I just do a small part, or do I need to build up to the entire pattern as I get enough stitches on either side of the repeat?

My blog is at http://pegotty/blogspot.com if you'd rather leave a suggestion there. Any help would be welcomed! Thanks all.