Hi Doug:
Welcome to RetouchPRO and thanks for posting your question. You didn't specify what version of Photoshop. I'll assume 7 or CS.
Here's my 2¢...
As tremendously functional as Photoshop is, including the new brush engine that was introduced in PS7, I believe in the long run you'll be disappointed with
PS when it comes to trying to simulate watercolor effects. As it was explained to me one time, Photoshop's underlying purpose is photo editing, not duplicating traditional art methods. Since then its been my observation and personal experience that for certain styles, watercolors among them, the results obtainable in this area from Photoshop come up short for those who have traditional skills and high standards.
But there is hope on the horizon...
Have you looked into Corel Painter 8? It's purpose is to provide digital tools that very realistically simulate traditional art in just about any style you can image... pencil, pen & ink, crayons, oils, pastels, acrylics, watercolor, chalk, charcoal, etc. From what I've seen if you have a tablet it does watercolors better than any other program on the planet. The brush options and controls are practically unlimited, including (for example) such things as at what rate digital watercolor strokes should run (due to amount of paint applied + gravity) as though painted on paper mounted on an easel. The learning curve for Painter isn't trivial, but functionally it's in a league of its own.
If you do look into Painter (
www.Corel.com), I believe Corel is still offering the version 8 upgrade that will install properly as long as you have Photoshop installed. You don't need a previous version of Painter for this less expensive upgrade nor do you need to get the standard (more expensive) version.
Although I didn't have any brilliant suggestions on the brush settings, I hope this gives you some food for thought.
~Danny~