flightdetent thank you for your reply.
The slat / flap failure and any subsequent flight planning with this failure prompted me to investigate this further. The FMS fuel predictions are not correct and in general you would use a fuel penalty factor . This penalty factor is a rather conservative one and if you only had a slat jamming at position one , then with some realistic calculations you could consider continuing to your destination if it's a rather short flight .
Realistic calculations are : Establish fuel required from level flight to top of descent using actual fuel flow . Establish fuel required from top of descent to touch down . The key is to find the right top of descent point with the existing configuration.