Well, it's not easy to find good data on this. It's not like you can calculate it either. The Russian territory in Europe is only about 25% of its total territory but that doesn't mean only 25% of the forest is.
Good idea but I wonder if you've allowed too much time? I scored 100% even though I didn't know much about it by guessing quite rapidly. Omit the obvious ones like the Vatican or Iceland and you can get through most of Europe in the time allotted.
Joke aside, that's why I still tried the Vatican, because they have some gardens, and if even just four or five trees in those gardens are close enough together, that could already be a few percentage of the whole area!
I think I actually guessed every single country in Europe except a few of the micronations. Generous time limit, rendering my 5 points pretty meaningless.