Can you adjust the needle position on your machine??
I run test strips. I cut 3 strips 1" by 3" long. I then sew all three strips together, press the seams, and then I measure the whole unit after pressing and it should equal 2.5" wide. Then I adjust my needle position to the right and make another test strip and repeat until I get the desired 2.5" width.
The act of folding the seam as well as the thread takes up space, add to that ... 1/4" feet are not perfectly accurate.
if you can't adjust the needle position on your machine, you'll have to use some other marker other than the 1/4" foot. some folks use a stack of post it's, some use tape or moleskin ... etc.
If you have the attachment and the hole for it, you can use a fabric guide. Here's a pic of one. These are terrific if you can use them.