Most FMSs will calculate it for you by changing the EFC time. If not...
Add:
- fuel required to get from the hold to destination
-fuel required to get from destination to alternate
-at least :45 minutes after the alternate.
-10% for error
Subtract that from FOB.
So-
You’re holding and have 10000 lbs on board. It’ll take you 2000lbs to get to your destination and another 2000 to get to the alternate. Min fuel is 4500lbs.
2000+2000+4500= 8500lbs + 10% = 9350lbs.
You have 650lbs you can hold with which in my jet is about 6 minutes, or not very long.