FitBit Calorie Adjustment OR Manual Enter Excercise Right?

If you manually log a workout, you will be asked for:
-Start Time
-Duration

MFP sends that information over to Fitbit. Fitbit then uses that information to "correct" it's calorie burn for the start time and duration you told MFP.

Then the adjustment math on MFP changes to avcount for the logged exercise. MFP increases it's estimated burn and the break down becomes:
Fitbit predicted calorie burn - (MFP calorie burn b4 exercise + manually logged workout) = new adjustment

Example:
Lets say your predicted Fitbit burn for the day is 2300 as of your last sync.
Lets say MFP estimates you burn 2000 calories a day before exercise.
So your starting adjustment is 300 calories (2300 - 2000 = 300).

Logged - 300 cal burn
Lets say that increased what Fitbit originally thought by 100.
So now we have:
Fitbit burn predicted to be 2400 after adjusting for logged workout.
MFP burn estimate becomes 2300 (2000 + 300 exercise = 2300) after logging exercise.
Now your adjustment is:
2400 Fitbit - 2300 MFP with exercise = 100.

So in this example you still burned 100 extra calories through daily activity. There is no double dipping, since the adjustment is a comparison of calories burned and nit related to steps tracked (unless you log for the wrong time or duration....then the math will be off).

Why are you manually logging workouts on MFP?

If it's the type of workout that is more accurate manual than say by HR calorie burn (lifting) - entering it on Fitbit has better accuracy because of the way they use the database.

But if it's not that - don't even worry about manually logging them, MFP or Fitbit.

Just make a wall post about your workout for others to see, with more details than you'd normally get.

Besides, with the constant syncing issues that MFP has with 3rd party trackers - the entering on MFP forces a sync to them, and then requires another back again.
Doubling the chances of a screwed up sync and bad data on MFP.

And the adjustment has nothing to with steps specifically - but calorie burn in total that Fitbit reports.

Might read the Fitbit FAQ. First section only unless you enjoy math and knowing the details of why.

http://community.myfitnesspal.com/en/discussion/10098937/faq-syncing-logging-food-exercise-calorie-adjustments-activity-levels-accuracy/p1

Unless it's cardio, which is just as good an estimate straight from the device as you manually entering it.

Unless you are on meds that you know inflates your HR for all activity - then the HRM should be turned off as it's totally going to skew results of calorie burn.

If not a HR-based device - then indeed you might be entering non-step based activity manually as workouts - lifting, swimming, rowing, elliptical, spinning, ect.
Walking, running, aerobic classes are just fine step based.