carriem, if your center is 18 1/2", your 1st border needs to be 3 1/2" by 3 1/2"unfinished instead of the 4 1/2" size unless you want to put just a fill in square in each corner. If you will make your round at 3 1/2" then your center will measure 24 1/4" after the round is attached and you will be back on track for the 4 1/2 - 6 1/2" sizes. It's up to you but I think you will find it much easier to go that route. "brat" and I have worked on both sizes in the other RR's that we are in and the 4 1/2" and 6 1/2" rounds come out more even and you don't have to do a bunch of math on each and every round for your quilt. If you need some help just ask and between the two of us we can figure that out, depending on what border you want to start with.