I'd strongly recommend replacing the stem rather than using a steerer extender-it's mechanically simpler, lighter, more sturdy, and looks way better.The stem calculator
phughes linked to is very useful, but you should also have some idea of how actually the bike should fit. Keep in mind that generally speaking bringing reach back will also be more upright. The steerer on that bike is as long as it can be within spec, and for the majority of riders for whom that bike is appropriate both in fit and in intention, that should be plenty high, but human bodies vary etc so do what's right for your body. If you've ridden road bikes plenty do what you know is right for you, but if not consider trying out a lower position than you're accustomed to--a lot of people have a knee jerk reaction to a forward fit because they have a fear of putting weight over their front wheel more than anything else. Consider posting some photos/videos of you on the bike to the fit forum, or actually go see a fit specialist.