Although we combine with internal scoring and manual review, as stated; If I was using MaxMind exclusively I'd consider 5.0 to 7.5 a good indicator of a possible fraudulent order.

This is based on their current riskScore system[1] (changing on January 1st, 2014) and 10 as an instant failure without review. Most orders will generate a non-0 score, however.

Another great tactic for preventing fraud is to never indicate an order has failed or a card hasn't been charged ('ghosting'), this is a tactic used heavily by Google for AdWords and other paid services.

Giving a clear indication of failure allows "carders" a way to easily figure out your detection algorithms by placing orders until one gets through, and share that information with others who will attempt to victimize your checkout process.

[1] http://www.maxmind.com/en/ccfd_formula