Thanks Barry,

"So the 2 lines at the bottom are showing the bottom angle and then offset 4mm this line is used to Boolean difference the inner to give the bottom 4mm thickness."

I don't fully understand this part. I used the upper line (marked yellow in my screenshot) to Trim off 4mm from the inner Solid. Then I ran the Boolean Difference operation to create the thickness. Is there an easier way of using the lines when working with Booleans?