Calculate "you save" to show on product page

I need to take the regular price of a product and subtract it from the special price value in order to get an amount to show as “You save: $xx.xx”.

I have tried to modify /app/design/frontend/MyTheme/default/Magento_Catalog/templates/product/price/final_price.phtml and
with no luck.

I am trying to avoid creating a custom module to do this but I’ve not found a way yet that works. If I need to create a custom module, I will.