Yes, indeed, it does have to do with the stagger between the wings

With the wings level, measure from the front of the top wing to the back of the bottom wing.

Not diagonally, but parallel to the ground. That dimension will be the used as the wing chord.

Let's say, FOR EXAMPLY ONLY, it's 20". 27% of that is 5.4". Measure back from the leading edge of the top wing 5.4" and that's the 27% point of the the total chord.

Get out your ruler, do some measuring and you'll find the spot that GP suggests is pretty good to start off with.