Hum, there is likely a few ways to achieve this. And it's literally been years since I've twiddled with these settings myself. I would try this though:
- Place your DashBoard under /sitecore/DashBoard
- Add a new Site Definition, looking something like this. Make sure this is patched in before "website".
loginPage probably needs to be taken out. But ideally, you should make sure your custom web application inherits some of Sitecore's base page logic, so as to use a seamless login from the Sitecore client to your application, and to benefit from Sitecore permissions. That is a story for a different day however.