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Ed has clarified your situation as to why the fuse has no influence on the operation of the pump. I also did not have access to a correct wiring diagram so my fuse position was not correct for your model year (I had to use a 73 diagram), but Ed has outlined what you should try to do--find the fuse terminal that becomes hot when the ignition switch is in the "run" and "start" positons, not the acc. Try terminal 15 first and if that is cold in the off and acc positions but becomes hot when the switch is in the "on", go ahead and use it. Your noticeable drop in voltage to the fuel pump is likely related to the wire being connected to an acc. terminal as, in some cars (not sure of Porsche), the acc power is shut down during cranking and restored once the switch is returned from the "start" position. Finding a terminal that is powered only when the ignition is in "run" and "start" should fix that problem as well.