I'm back ,had to visit local A/C dinner, it is hot. Ok I'm willing to take a leap of faith, really don't understand 12 7/8 square. I'm thinking 3-4 inch HST's to circle (diamond) my 17.5 inch block. I'm thinking 12 7/8 for the corner after I finish my HST's. I'm trying to figure out how big my HST's should be. I'm thinking each round of HST's might need to be bigger