There is no absolute standard for the margins of a book; it’s really a matter of aesthetic balance between the dark type area and the white surround.  Bear in mind that when we look at a page we also see the page facing it, and that has to come into the judgment of balance.  In general we can say that “correct” page layout will end up with the gutter margin being the smallest, the head margin next, the fore-edge next, and the foot margin largest of all.  This enables the reader, in theory, to hold the book with a thumb at the bottom of the page without obscuring any of the text.  Of course de luxe margins, like de luxe anything else in book manufacturing, are now a thing of the past.  But still the proportions should be maintained if possible as long as we care at all about the look and feel of the book.  For those who crave a formula a relationship of 1½ : 2 : 3 : 4  is good old conservative guide.

It’s probably safe to say there are no circumstances under which the gutter margin should be less than ½”.  So with a foot margin approaching 1½” you’re probably going to be told you’re wasting paper if you persevere with this formula.