Quite simple really...when down to alternate + reserves fuel, divert.
Seat 0A,
Not familiar with the alternate? Well, as PIC you certainly should be, 'tis a company requirement where i've worked.
Alternate doesn't know you're enroute?
That is what ATC is for....
And especially for the long haul folks, to overfly a suitable enroute alternate KNOWING that you will be really short upon arrival at destimation is, in my opinion, a terminating offence...as in, find another job.