For starters, throw out your d(effective) or average diameter. The pressure area or sealing area will be defined by the transition of pressure from high side to low side. It would help if you defined which is which. There may be a pressure gradient between 2 radii that needs to be considered. I see there is a groove in the face of the underside of the poppet outside d2. Is this empty or does a seal lie in there? I don't see how it could seal at d1 with the x-shaped stem crossing the d1 diameter. You should consider that it might be sealing at the OD or at one of the lips of the groove. Is there a spring involved in the assembly on either side of the poppet?