I know the forum rules prohibit promotion of commercial items. Even though Nova Montgomery's seam guide web site links have been posted here I expect this post will get deleted even though it addresses the original question. But I'll try.
The question asked is how to get a scant 1/4" seam on a Featherweight. Maybe a better question is how to get a REPEATABLE scant 1/4" seam, since most people don't start and finish one project without doing other sewing on their machine in between.
Last year when I was in Houston I saw the Ideal Seam Gauge and guide made by Steve and LuAnn of Sew Very Smooth. I thought it was really cool but thought that they could make some improvements for use on the FW. I stayed at their booth for 45 minutes watching Steve demo it and then went back later for another 45 minutes. The result of all that conversation was the Featherweight Special Edition of the Ideal seam gauge and guide.
The beauty of this system is that it gives you a repeatable seam allowance from 1/8" up to 1-1/2", including a scant 1/4". Since it measures from the needle you get the exact same seam allowance today, tomorrow, next month, or next year, so that your pieces always line up.
http://youtu.be/XHtA-eEuWpI
For those concerned with putting an adhesive on your Featherweight's finish, not to worry. The guide uses an engineered plastic strip that is tacky on a smooth surface - not an adhesive. We have tested it over and over on several machines in our shop and sold almost 100, and have not had any issues. You do not want to place it on your decals though and the guide is designed to not touch them when you have it properly located. When the strip gets linty and stops sticking you just spray it with blue window cleaner and wipe that off. When it dries it goes back to sticking to your machine.
All of the customers I have talked to love them.
The only gotcha is that when you are working with a normal seam allowance like scant or regular 1/4" the guide sits on both the needle plate and the bed of the machine. If the needle plate on your machine sits really tall compared to the bed as it is on some later machines, machines with a lot of dried oil and gunk under the plate, or the aftermarket graduated needle plates, then it will leave a gap under the guide in front of and behind the needle plate. In our experience it works fine on most machines.