The current data set is dummy data. In the actual we all the countries within the continent. So MENA will not just have Saudi Arabia, it will have other countries like Bahrain, United Arab Emirates, Oman etc.
So I do not think that logic would work unfortunately.
One other thing you can do is group all the Countries in to one single country. like this.
Give the group names proper country names so that it will be a geo field. Now you can use this Country (Group) to calculate the Account details.
As the fixed calculation in my previous message, instead of region, use Country(Group). I think this works fine. Please let me know.