Using an 8" springform pan in a recipe for a 9" pan

Making Lucy Mercer"s lemon cheesecake (January 14, 2012) in which she uses a 9" springform pan. I have an 8" pan... do I just use as much of the filling as I need (what, 2/3 of it, 3/4 of it?) or try to reduce the whole recipe (again, by what?). Or am I over-thinking this! Thanks for the comments.