Fitbit

It varies from person to person. The only way to know is to trust it for 4 weeks and see what happens to your weight in that time.
On your fitbit profile page (website, not app) you will see a 30 day average burn and 30 day average intake.
30 day Avg Burn - 30 day Avg Intake = 30 day Avg deficit or surplus
30 day avg deficit or surplus × 30 = Total deficit or surplus for the 30 days
Total deficit or surplus/3500 = expected loss or gain

Did you lose more than expexted? Then its underestimating or you are overestimating your intake. You can most likely eat a bit more and be okay.
Did you lose less than expected? Then it's overestimating or you are underestimating your intake. Either tighten up your logging, figure out the percentage it was over by and don't eat that percentage of your adjustment back, or do both.
Did you lose what was expexted? Great. No adjustments need to be made.

When I used the flex, I found that it underestimated my calorie burn by about 200 calories per day on average.

I've been eating what MFP tells me to, + eating back most of my exercise calories as calculated by my FitBit flex, all summer and have lost weight at very nearly the rate I had set (1kg per week). Based on that, I think the calories burned as the FitBit calculates them are pretty accurate (for me). (But I totally agree that the best way to test it is to try it and see what happens!)

As a way to trick myself into burning slightly more calories than my fitbit is recording, though, I do the following:
-wear my fitbit all the time (for accurate step calories)
-log exercise that is not captured by the fitbit at all (namely biking and swimming, 1-2 times per week)
-any other exercise I do just gets logged by the fitbit as steps, even though I'm not getting "full" credit. Any strength exercises, yoga, and most cardio is actually better exercise (burning more calories) than the fitbit is capturing, but I intentionally don't log it outside of what the FitBit captures. I think this extra exercise probably helps me break even.

I hope that helps!