Just to clarify: Are you saying that the outside borders are 6" too short in one direction? So you want to add one pinwheel at the end of each outside border? You only need to add 2 pinwheels total? (Sorry - I am symmetrical thinker and I would have to take those borders off and add pinwheels in EACH of the 4 corners - but that is just me).
It sounds like MTS' instructions are spot on. You need your HSTs to be 3.5" (unfinished). WHat method do you plan to use to make the HSTs?
When you sew the 3.5" HSTs together, then you will lose 1/2" in the center (3.5 + 3.5 - .5 = 6.5) When all of the HSTs have been sewn into a pinwheel block, it will be 6.5" That needs to be added to your border.
MTS, she said that the border is cut 6.5".