If you are familiar with the notational practices and with the repertoire, you will be able to figure out the proper interpretation.
Consulting the Laon manuscript can help a great deal in this regard, since it tends to be more reliable when it comes to rhythm. But this is definitely an area where a little bit of knowledge can be dangerous.
I might add to this that Metz was at that time esteemed higher than St Gall in terms of chant and more faithful in preserving the tradition.
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