In Search of the Perfect 1/4"

I've completed a few quilts, but I have not mastered the most basic element of piecing, the scant quarter inch. For a lot of the projects I picked, it didn't matter, because as long as I was consistent, it was fine. However, that is no long-term solution, so I'm experimenting to find out what adjustment or feet or pressing method will get me the perfect quarter inch.

I'm using the method of sewing together 3 strips of 2.5" across, then measuring, and hoping the end result is perfectly 6.5". For each foot / needle-position combination, I'll first press to the side, measure, then press open and re-measure.

Test 1: Zigzag foot at needle position 3.5.

Pressed to the Side: Just shy of 6 5/8
Pressed Open: 6 5/8

Test 2: Zigzag foot at needle position 3.3

Pressed to the Side: 6 7/16
Pressed Open: 6 1/2

Test 3: Quarter-inch foot (no adjustment)

Pressed to the Side: A few hairs more than 6 3/8
Pressed Open: A few hairs less than 6 1/2

The closest was using needle position 3.3 and pressing open. It seems when I first press to the side, it's not very accurate, but if I press open and *then* to the side again, it's much closer. That's a lot of work, but worth it if I want more accuracy and nesting seams as well.