'Scant' - when referring to cooking measurements or seam widths - is just a 'tad' or 'smidgen' less than the stated measurement.
Seriously, though, I think in many cases, it means to use an exact 1/4 inch - or perhaps a 31/64 inch seam - or a 63/128 inch seam - there are so many variables, that one has to do a little practice run for most projects.