I'd say 40-50 is moderate. 30 shouldn't be too much of a problem most of the time if you nail onpage optimization and can get some links. Of course, depending upon budget, time, experience and resources... 40-50 can sometimes be difficult to achieve.
Rather than looking too much at the difficulty score, I'd be looking at the metrics for the top ranking sites and gauging whether or not it's reasonable that I can beat those metrics. Keep in mind, you may not want to pay as much attention to the metrics of an exact match domain or international brand name because you can't duplicate either of them
Pull an advanced keyword report for your top phrases, download them into excel and then add some metrics to them. Once you're done, the big question will be "is it realistic that I can beat these metrics with the allocated (time and money) budget?".
If you can, then I'd pull a "Top Pages Report" with OSE for each ranking website and see what kinds of content and tactics they've employed to get those metrics that are helping them to rank.
Armed with this information, you should have a very clear picture of what needs to be done to win, where to start and if it's realistic or not.
Does this help Zachary?