Each machine will give you different caloric results depending on many variables (speed of each rep, amount of weight, your form, etc). What I have found to be accurate enough over time is to manually log my workouts (non cardio) by selecting 'weight lifting (free or nautilus or universal type...) light or moderate effort.
What I was taught in school (I'm a personal trainer) is that you can generally assume about 100 calories an hour for a typical weight lifting session. I spent some time adding up how many minutes I spend exercising vs walking around for cool down or doing something like stretching and that is how I log it. So, if I'm in the weight room for an hour I might log 20 minutes of 'weight lifting' and let the fitbit get the rest of it. I have also found that doing free weights, lower body (kettlebells, squats, etc) record accurately without me needing to manually log, but upper body or machine work needs to be manually logged.
