function calculate(assemble, file, target) {
  compute(file.data, target, function () {
    var opts = extend({}, assemble.options, file.options);

    // destination directory for the file
    var dest = path.resolve(this.dest.dirname || process.cwd());
    target = path.resolve(dest, opts[target] || target);

    // look for `opts.assets`, fallback to `./assets`
    this.assetsPath = relative(dest, target);
    return this.assetsPath;
  });
}
Calculate the path from the `assets` or `public`
path define on the options to the destination
file.