Nova Montgomery sells a seam guide for the Featherweight that screws in. I received mine a few days ago and am impressed by the quality. I would be hesitant to to stick something to a FW, especially as mine has a really good finish on it.
The quarter inch foot, depending on the machine is not always a quarter inch. You also want to be able to train your eyes in front of the foot rather than on the foot or on the needle. Right now, I'm looking at where the fabric meets up against the feed dogs.
I do recommend the tests for a true/scant seam. The one I use is the one I learned from Harriet Hargrave's Quilter's Academy. Cut three short strips that are 2 inches wide. Sew all three together and then measure to see if your center strip is 1.5 inches. If it is not, adjust your seam width. I have a ruler that is 1.5 inches wide - so if I can lay it on the center strip and it fits perfectly, I know that I've nailed the seam.
This is something you have to test on a regular basis. Bonnie Hunter pointed out in the class I took on Saturday, that the scant quarter inch is dependent no only on seam width - but also on the type of fabric and weight of the thread you are using. And also on how well you press your seams.
Cheers, K