Yes you are closer. I count around 110 fast days but they would not have had meat every day nor fish every fast day so it still would come out more or less to 20%. Remember every Sunday was feast day, likely on that day they would have had meat.
And I think they were not allowed to eat what they called “white meat” - eggs and cheese - on fast days. The liturgical calendar dominated their lives and often in very beautiful ways. Christmas was almost a month long in England during the 15th Century.
True. Meatless days doesn’t mean you’ll have fish, just that you won’t have meat. And days you’re allowed meat doesn’t mean you can afford it, either. Except for the really rich, practically everyone’s diet was heavy on grain and dairy in that time period.