CE560XL Captain reports being issued a crossing restriction; which is entered on the VNAV page of the FMC. The crew discovers too late that the calculated top of descent time is not correct and the crossing restriction is missed. The crew learns from the manufacturer that an altitude entered on the Plan page will provide the top of descent time and not the crossing restriction entered on the VNAV page.

Narrative

While flying at FL430; 90 nautical miles west of the Albany VOR and flying direct; my co-pilot and I were given a crossing restriction to cross 60 miles west of the VOR at FL360. My co-pilot entered into our long range navigation unit; VNAV; the crossing distance and altitude given. It confirmed it would be approx 6 minutes before we were to begin our decent at 2000 FPM to the crossing distance and altitude. After approximately 3 minutes I mentally computed that this could not be correct and the VNAV portion of the LNAV was giving us an incorrect decent time. I began an aggressive decent when the Boston controller came on the air and asked if we were going to make our crossing. The co-pilot remarked we were going to do our best and the controller replied we were going to have to do better than that and gave us a 30 degree vector to our left and called another aircraft to turn 30 degrees to their left then continue our decent to FL330 which we did. We were handed off to another controller who gave us direct to the Albany VOR and the rest of the flight was uneventful. Once on the ground we reviewed what happened and worked through the key strokes and directions to enter the decent. We were unable to duplicate. Once back home at we contacted the manufacturer and asked if they had seen this problem with the scenario we were given. They remarked they had seen this before. It was a problem with some units and advised us on how to enter information in our unit so as not to create this scenario again. This info is not in the operators manual.