1. 610 now refers to 682 version 2
2. The pressure you need to define is the pressure in the stuffing box/seal chamber. This process fluid pressure is between the discharge pressure and the suction pressure and depends upon the type of impeller used in the pump. Many OEMs put the stuffing box pressure on the pump curve, if not check with your pump or seal vendor.
3. API plan 53 requires that the barrier fluid used between the inboar and outboard seals to be 25 psi above the stuffing box pressure. Again, your seal rep is likely very experienced and would be happy to help.