Well getting a true 1/4 inch can depend on alot of things. So here are a few things to think about.
What kind of thread are you using?
What kind of needle are you using?
Do you use a straight stich plate?
What fabric are you working with?
In my case I was having issue with getting a true 1/4 seam, so when I had a gal at my LQS look at it with me, here's what helped me get an accurate (as long as I lone it up to the 1/4 on my stritch plate.) ")
**I'm using Aurafil thread, the properties of Aurafil is alot less hairy and therefore goes through the fabric easier.
**I'm using a Universal 90 needle with 100% cotton Fabric. (With Batik fabrics Sharps are better)
***The straight stitch plate is great because a zip-zag plate can pull the fabic down into the plate at times making your 1/4 seam allowance off at time.
After making those adjustments I've been pretty good about keep a 1/4 seam.
Also....what the heck is a scant 1/4?? Isn't the same as a 1/4?