How well armoured was the Roman army?

[Robert Vermaat]

Was the late Roman army more well armoured than the army of Principate percentage wise?

Impossible to say as we lack detailed knowledge. Only by the late 6th century we hear of advice to provide armour at least for the front and back rows, but no-one can tell if that was specific for that period or earlier. Claims by a 4th c. text (Vegetius) that the Roman army had dropped all armour have been amply prroven to be untrue.

Speaking generally, I would say that the Late Roman army command would have tried to provide as much armour for their troops as the Principate army command would have. Both infantry types were mainly of the heavy sort, although it has been argued that the Late Roman infantryman had a more versatile role. Meaning, he would not have worn heavy armour on all operations (such as search and destroy, commando-style night raaids etcetera), tasks that were more left to the auxilia during the Principate. In a pitched battle though, both would have had a lorica of some type, a helmet, and probably armour on at least the right arm and the left leg. I think.