I think you are asking what size does your center panel need to be?
So in reading this, I worked backwards. However, I had to assume that your Piano key border was going to be 6"W
A Twin size mattress is usually 39"W X 75"L
Based on that size with a 10 inch drop off of each side, your finished quilt needs to be 59"W X 95"L
Working backwards, Subtract 12" from each figure for the piano key border.
Subtract 18 for your "4, 3, and 2" inch borders, and that leaves you a rectangle size of 29"W X 65"L for your center panel.
If these measurements are not correct, then you still need to start with the finished size of the quilt you want and subtract from the outside to find the measurement of your center panel.
AND, do not forget to add your 1/4 inch seams all around either or your quilt will be smaller than planned.
Hope this helps, and good luck
Susan