The full path to a website's home dir is always /var/www/{websiteId}. You can get the websiteId from your dashboard, it's the long uuid in the address bar.
However in almost all cases you can just use a relative path, either relative to the home dir or the current dir. For example, if you had a cron script in public_html/cron.php you could use ~/public_html/cron.php or just public_html/cron.php as the path, since cron will always start in the root of the home directory.
