The tough part about figuring out an Arthurian chronology is getting Merlin's prophecies to work out right. When Merlin convinces Arthur to let Lot and the other rebels escape in the first year of his reign, he does so by convincing Arthur that Lot will not threaten the realm again for three years. He's very specific. If Arthur's reign can be dated to 410 based on internal evidence, that means Lot is not finally defeated until 413. But it seems to happen in mid-412.
One more big year to go and then I will not worry about any more chronology for this manuscript.
Read 411 AD here and 412 AD over here.