I've been told a "scant seam" is one needle width shy of the 1/4". So here's how I'd do it. I'd use a bed mounted seam guide and a plastic ruler. I have several that are so thin a sewing machine needle will punch through easily.
Punch a hole one needle width shy of 1/4" then use the ruler to set the fabric guide.
I use the ruler method in all cases where I do not use my 1/4" foot with the edge guide. It's very easy to set your fabric guide that way.
Joe