Thank you!
>> leaving one cell empty
It really does help to be able to see the mat clearly. If you can see it between the rim and tray edge, and between two cells, that's fine. Or if you cut your insert or plug trays so that you can lift just one row and see the mat, that's good.
If you can see or touch the mat, you can tell right away how dry the soil in the bottoms of cells is (are?) because the mat and the bottoms of the cells or pots will be almost equally dry. That helps me avoid over-watering, since I have no excuse as long as the mat is wet or even damp.
