These instructions were in the 2010 BOM Ruffled Roses:
The Math of Floaters
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Let’s take a look at the math of floaters.
Starting at the beginning, what, exactly, is a “floater”? It is an unpieced border strip, set into a quilt. It
can add an accent fabric to the quilt top – or it can add to the size of the quilt top so that the next border
will fit precisely, without chopping off any pieced units in it. For example, if you have a 12” block and
want to add a border of 5” squares around that block, three 5” squares would result in a 15” length, so you
would have to cut off one of the squares to make it “fit” the 12” block. Alternatively, you could add a
floater so that the dimensions of the 12” block and the floater, together, equal 15”. How do you do the
math? Take the length of the inside unit (12”) and subtract it from the length of the outer unit (15”).
Divide that number (3”) in half – giving you 1-1/2”. Your finished floater would thus need to be 1-1/2” in
width… but don’t forget that when you cut the fabric, you will always need to add two quarter-inch seam
allowances (i.e., a 1/2”; this includes a 1/4” seam allowance on both sides of the floater strip).