CE560 CREW INCORRECTLY PROGRAMMED 'FMS' AND UNDERSHOT XING RESTR.

Narrative

WE WERE LEVEL AT FL350; DIRECT TO MRB VOR. ATC GAVE US A CLRNC TO CROSS 80 MI W OF MRB VOR AT FL250. I PROGRAMMED THE VNAV DSCNT INTO THE FMS AND STARTED DOWN AT THE RECOMMENDED RATE. ABOUT 75 MI W OF THE VOR; ATC TOLD US WE WERE ALREADY INSIDE 80 MI AND TO EXPEDITE DOWN TO FL250. AT THAT POINT; WE WERE AT ABOUT FL280. I EXPEDITED THE DSCNT AND WE ULTIMATELY REACHED FL250 ABOUT 65 MI W OF THE MRB VOR; MISSING THE RESTR BY 15 MI. I BELIEVE I PROGRAMMED THE FMS INCORRECTLY FOR THE DSCNT. THE INITIAL RATE OF 1000 FPM SEEMED SHALLOW. HOWEVER; THE DSCNT CLRNC WAS ISSUED AT 'PLT'S DISCRETION;' LEADING ME TO BELIEVE OUR DISTANCE FROM THE 80 MI FIX WAS FAR ENOUGH TO JUSTIFY THAT RATE. MY ERROR WOULD HAVE EASILY BEEN DISCOVERED HAD I DONE 2 THINGS: 1) ASKED MY COPLT TO VERIFY MY INPUTS TO THE FMS AND 2) CALCULATED A DSCNT RATE IN MY HEAD USING THE DME READOUT FROM THE VOR. NORMALLY I LEAVE THE VNAV PAGE OF THE FMS DISPLAYED SO THAT I MAY CHK MY PROGRESS. THIS TIME HOWEVER; THE PAGE WAS NOT DISPLAYED BECAUSE MY COPLT WAS NOW USING THE FMS TO SEND AN AFIS MESSAGE TO THE ARR FBO. THIS SHOULD HAVE ENCOURAGED ME EVEN FURTHER TO USE THE DME READOUT TO CHK MY PROGRESS. I ALSO SHOULD HAVE OCCASIONALLY INTERRUPTED MY COPLT SO THAT I COULD CHK MY PROGRESS ON THE VNAV PAGE OF THE FMS.