Are you saying that the leg runs empty for 2 minutes/truck dumped?     Sounds like a reasonable number to me.

Where you lose me is that you claim you're only losing 250 bph of leg capacity/hr.

You're dumping more than 1 load/hr unless you are absurdly overweight.       If you're hauling close to legal and dumping 6 loads/hr you lose 12 minutes of leg capacity for every hr.     

12/60 is 20%             7500 * .2 =  1500 bph of lost capacity

Please correct me if I'm reading your numbers wrong.