scant 1/4" is a guideline. How the unit measures when finished is the goal.
There are many variables, one important one is where you guide the fabric. A 1/4" foot guarantees nothing, where the fabric is, is key. Another is pre-pressing.
Fabric fresh from the store seems so flat and smooth. But cut from it and then do peicework and it can grow or shrink with pressing.Depending also on if it was pre washed or not.
Another key is knowing where you decide to hold the ruler lines. Be consistant. My lines lie right On the free edge.
We all have to make our own decisions,...and then stick to them for consistancy.
I have dozens of 9-patches I've made over the years on different machines. I wish you were here, you'd be sewing perfection before your tea was gone!
Might seem like a pain to bother knowing your 1/4" is right but nothing in this world would make me trim blocks when finished.