You can always calculate when a project finishes and donate the second a new one opens. There are also lots of fleets that are in desperate need of contributors where it takes over a day to complete a project.
I went today from 128,000 fleet credits to over 164,000 thousand and I have 5,000 unrefined dilithium left. That's a gain 36,000 fleet credits in a day.
Took me awhile, but I think I've mastered the art of earning fleet credits,,except I don't use alts to farm it.
What I do is Officer reports which gives both 50 fleet credits and 960 dilithium for playing spotlight foundary missions, pick empire betrayed foundary mission. The thing you got to realize is that officer reports require 30 minutes between officer reports, but the count down starts right away, so by the time its over, the timer is like 12 to 15 minutes before you can do it again. Also note if you start an officer report before leaving the game for a time, you can basically do back to back officer reports.
and then choose all those space PVE qued events thingies that give fleet credits, do one of those and by the time your done, you should be able to do another officer report.
If you have a couple minutes to wait, pop over to Klingon/Starfleet Academy and go to the Orion, Nausicaan, Gorn and Imperial Personal officers to do duty officer missions for more duty officers, and even spend 1000 dil for the general recruitment.
Also if you have the contraband trade it in for 2000 dil doff mission with the security officer in the great hall.
Seriously I don't even have to leave Qournose anymore.
Also doffs give at least 300 fleets credits each.
Plus some missions will use stuff like stabalizers, particles, industrial cells as well.
If you net 200 fleet marks and dump them on a project that's 10,000 fleet marks right there.
Now all you need is a moderate sized fleet, one thay advances at a fair pace, but not so fast that there's no room for you.
My fleets just the right size I'm useful, but we get projects done at a fair rate.