Getting path to project directory when project file stored elsewhere

I want to use something like `$my_project_directory/public/index.html` in a build system file but as I store my project file in a different directory $project_path points to that directory instead. How do I get  arefernce to the correct directory?

project file

```

{
   "folders":
   [
     {
       "path": "/Users/wheresrhys/Sites/jigsnreels/dev"
     }
   ]
}
```

build system

```

{
  "cmd": ["open", "-a", "Google Chrome Canary", "$project_path/public/index.html"]
}

```