If you are planning to use it in your theme, you can do the following:

define( 'SCPT_PLUGIN_URL', get_template_directory_uri() . '/super-cpt/' );

However, I do not favor this approach as it prevents the plugin from being updated.

This seems pretty neat.
http://tgmpluginactivation.com/
Haven't tested it on my own code yet.
Seems to make plugins super easy to maintain - paste your plugin archive in your theme's folder, call it from a function that uses a hook created by this library.