Fridays are 1/7 of the year. 40 days of Lent. 40 days of Advent. Vigils of the major feasts such as Annunciation, Ascension, and All Saints.
A little math on the back of an envelope gives me over 1/3 of the year as meatless days.
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.