Wide bindings can easily be done using a standard ¼" seam allowance and that way they do not change the block size or appearance of the top at all. For instance, any points on the edge of the top will remain on the edge of the top. You will not need any special math or be forced into using plain borders unless you want to.
Do not trim the backing and batting even with the quilt top before sewing on the binding. That is the only thing you need to do differently. Sew the binding on as you would normally do, mitering the corners as you would normally do, and joining the ends as you would normally do.
When all that is done, trim the batting/backing to the width of your binding by doing the following: From the binding seam line, measure over an amount equal to the desired finished binding width and cut off the excess batting and backing. (It will not be even with the raw edge of the top and binding.) Then fold over the binding and attach it to the back as you normally do.
The batting and backing fill the binding as they should, there is no change in the pattern on the top, and there is no need to have plain borders because of an extra-wide seam allowance.
The only math you need for double fold (aka french fold) binding of any width (using ¼" seams) is this: The width to cut the binding strips = 4 times the desired finished binding width plus ½".