Rita, shouldn't it be 2 borders 3x17 1/2 and 2 borders 3x12 1/2?
The way I calculated it was to, first, take away the 1/2" seam allowance, then divide what is left by 2. Then I add the seam allowance back in. So, for example, if the first full-size border is 5 1/2 x 34 1/2, take away the half-inch seam allowance from 5 1/2, it leaves you with 5. You divide that 5 by 2, which gives you 2 1/2. Now add the seam allowance back in and you have 3.
Now do he same for the length of that border 34 1/2. Minus the seam allowance is 34. 34 divided by 2 is 17. Add back the seam allowance and you have 17 1/2.
Anyone else want to pipe up?