If the recipe says 2/3 cup of butter to make the caramel, it is advisable to count the little lines on the paper the butter comes in, and then do the math to convert Tablespoons to cups. Really.
If you get too happy and just wing it, your Turtle Bars will have a sizable layer of butter on top of the caramel layer, which will percolate down through the shortbread layer as they bake. They will be greasy and even richer than usual. This will make it impossible for you to strike that balance between feeling like you ate enough Turtle Bars and feeling like you are going to die.Really.