*15fps is plenty for survielence, once you get used to viewing it, even 10fps works just fine.
*Turn OFF smart codec in the cameras, that is propitiatory and only works with Dahua NVRs'.
*You don't need 10gb reserved for alerts. Alerts are just markers in the recorded file, set it to 0, unless you have "Save alerts as Hi-Res inabled, which you don't need.
*Don't change settings until you understand why you would want to change them.
*Shorter make times will increase the amount false motion detection, 1.0 sec is a good starting point, then go a 10th or 2 higher and test with the Motion. You WON"T get this figured out in one afternoon.
Setting Object detection and contrast more sensitive with increas the false motion dection. You set for your situation, by making a change in settings, then testing. Read the BI help file. And this:
Optimizing Blue Iris's CPU Usage | IP Cam Talk