Tell your captain no. That is why you are there as an FO. To make sure your captain has a good extra dose of judgement in the airplane. The captain says "ehhh, we can make it in, no problem." You are there to say "no." If you're the Captain, then perfect refer to above bolded item.
Since when does the FO get the final say?
If you're going to be employing those tactics with your captains, I recommend the following:
1) You had better be 100% right.
2) You better not be on probation.
3) You better have a supplemental source of income.
I'm gonna vote for germb747 on this one.
I've been in both seats, low time and high(er) time. I've been involved in the training/checking process from the giving and receiving end.
For all my experiences, yes, as an FO you have a significant input into the CRM modeled decision making process. Varying levels of prior aviation experience also are considered as a right-seater makes inputs. However, to think you "give the captain an extra-dose of good judgement", I hope is a improperly spoken thought. A Captain should already have an extra dose of good judgement.
One thing I've learned, is that no matter what I think of the Captain, there is a reason they're still around, as macbre as the thought might be. It's usually because they know not to get in above their heads. Crews historically arise to the occasion when thrown into dire occasions outside of their control. The recent Kalitta accident is a great example. They lost 2 engines right after takeoff, at night, in the mountains, in Columbia. The hull was lost, but the crew made it off the machine (I'm not entirely sure of the condition of the FE - I'm pulling for him though).
Have I caught mistakes a Captain has made when I was an F/O? Sure. I've had more of my mistakes caught when I was in the right seat than I caught though.
Did I have F/O's save my bacon when I was the Captain. You bet. They had cold beer too. Was I an unsafe Captain? I really like to think not.
In any event, everyone's goal is to fly safe and make sure you get home at the end of the day. Everyone realizes that mistakes can be fatal in this job, and unfortunately some have suffered the consequences of bad judgement. It can happen to anyone, which is the reason we have training progams at all levels of aviation to mitigate these types of error chains.....
Just $5.00 of my $.02....