I use the two toed foot and know that if I go about 1/32 over, I'm good. No idea what seam allowance that ends up, but the part on the other side measures what I want. I prefer to be able to see a bit of fabric past the foot when I sew, so I'd never want to get a true 1/4" foot that needed to completely cover the fabric to get the seam correct.
I keep my pressure on the light side of the middle. I've had mine for over 2years and never tried FMQ on it yet. Sure do love it for piecing!