The center of the measurement window is the pea sized circle in the middle of the glass screen of the device. It is the most sensitive area. You'll want to make sure that area is filled with flower only when completing a measurement. It is important that the well grinded sample covers the measurement window. If the sample is not evenly mixed then the measurement from different points may give slight varying results, ie) if you stir the sample in between multiple measurements. The finer the grind, the higher the accuracy. The more homogenized the sample, the better due to the variability within a flower sample.