https://gregobase.selapa.net/chant_img.php?id=7376

We start on sol and use sol-la-si/ti.
We have two finalis: sancti ends on sol but amen ends on la.
The recitation is la.
Hypophrygian, I think, has recitation tone la with mi as finalis. But I cannot find the mode for this simple melody.
In modern music we would say that Signum Crucis starts on Do abd end on do with amen ending on re which is not an ending tone.
What practical things can you tell me about this to me as a person who just started learning this? Where do one study this in depth?
I would say A-men rather than a-MEN. The second is used in the tune I think.