Most often, quilts are made to the size of a pattern. In this case it had to be made to the size of the bed: 104".
Quilts shrink in size during longarming. The thicker and fluffier the batting, the more the shrinkage. What you start with can easily shrink 1% to 3%. When you use fluffy batting like wool, shrinkage can be 3% to 4%.
To take this into account you can start by saying "what if..." and calculate like mad, ending up in "analysis paralysis". Or, you can add a margin for error and simply make your outer borders larger than they will be at the end. In our case Ruth had the overall quilt size [before quilting] to be 110" for the ending size of 104".
After all of the stitching was done, she trimmed the quilt to 104" square.