Kaye

I hope that someone can/will help me with calculating Sashing and cornerstones for a block swap quilt that I will be making. I will have 12 squares that measure 12 1/2 inches before assembling. I plan to use a 4 x 3 array with 3 squares across and 4 squares down. How wide should the sashing be? I plan tp apply the vertical sashing first to connect the rows of three squares so next i need to know What length do I cut the horizontal sashing pieces to attach to the corner stones so that it will fit perfectly with my rows of squares and Sashing?

I hope this all makes sense! I usually follow a pattern and someone has already done all of the math!!!

Thanks!