I don't rely on a foot. I use a strip of heavyweight moleskin and butt my fabric up against that as I sew. My seams turn out much more even that way.
To measure the scant 1/4" for moleskin placement, I place my favorite cutting ruler under the presser foot, make sure it is positioned fairly straight front-to-back, move my needle position one place to the right (with my older Bernina I have to do this in order for the moleskin to clear the feeddogs), lower the needle so that it touches the ruler just to the right of the 1/4" line, and lower the presser foot to hold the ruler in place. I have already cut the moleskin into strips using a ruler and rotary cutter, so I just remove the paper from the back of the moleskin and carefully position it so it is butted up against my ruler edge.
Although I often use just a 2" or 3" long strip of moleskin, my preference if I am going to be sewing long strips together is to make the moleskin strip even longer, so it is guiding the fabric long before the fabric actually gets to the needle.
Anyway, with this method I don't have to force my eyes to constantly look at the marking on a foot; as long as the fabric is butting up against the moleskin as it feeds, I am going to get a good seamline.
As someone else mentioned, thread thickness can affect seam width. Once you have the moleskin positioned, it's a good idea to do a test of 3 2-1/2" strips sewn together so that, when ironed, you are sure the finished measurement is exactly what it should be.
Moleskin is available in the foot section of pharmacies. It is a cushioning product sold for placing on heels, etc., to prevent blisters or protect a blister from rubbing.