I measure the fresh untreated spores." spore size varies with the treatment, especially the mounting medium."
I have now measured fresh spores from another part of the leaf with a maximum length of 95µm (P. interjectum).
So basal cells 2-3 and maximum length of spores 95 µm, two factors indicate P. interjectum.
I liked the name "Engelsüss" better than "Mittlerer Tüpfelfarn", such is life, it's not always sweet.