Hi Sam,
Wasn’t possible, but I’ve just added it to the plugin. After you update to new version 1.3.1, you will find new Free shipping minimum order amount option in methods’ settings. Unfortunately, this option is available in Pro version only.
P.S. If you are calculating shipping cost based on order cost, you can also use [costs_table] shortcode for minimimum free shipping order amount, e.g.:
[costs_table prop="cost" table="1-10|10-5|20-0"]
Hope that helps. Please let me know if you have any questions.