Eat HRM + fitbit cal to maintain?

Make sure you're accurately logging your exercise of mfp with the proper start times and duration. This cancels out calorie calculations for the same time period on the fitbit, so you aren't accidentally double logging calories.

I only got my fibit one in January, and prior to that I had a hrm for more than a year. My progress was very slow while using the hrm and I've come to notice something. On average I burn 2400 calories a day according to fitbit. On days when I do my long runs, I'm generally more lazy the rest of the day and actually end up burning less overall for the day, even with the added exercise. So I've stopped logging any step based exercise from my hrm, otherwise I'd be eating more calories than I actually burned for the day. It's so easy to see now why I was struggling so much before! I log all food on mfp, and have my calorie goal set to 1690, but I closely follow fitbit's calorie meter and food plan.

Now that you have both tools, you too will be able to track your trends and see where you need to make adjustments. If its at the end of the day and you aren't hungry, just think of them as bonus calories to spend throughout the week. 300 cals over your normal goal might give you an initial gain until you adjust. But if you continue to see gains, just drop a few calories again.

It's kind of an experiment when you get to the maintenance part. You log everything the best that you can and then see if you are maintaining at the end of the week. It's usually good to start guessing on the low side of calories at first and then add calories if you see you are still on a losing trend. It's so much harder to maintain than it is to lose because when you're losing, any loss is a win, so you just have to be at ANY deficit, but with maintaining, it's more of a juggling act. Be sure to consider water weight gain due to activities, sodium, and shark week. Sometimes the scale will go up 3 lbs in a week, and it discourages people (I am one of them), but you just have to keep working at making your numbers fit because everyone is different and MFP is only a guesstimate.

I would suggest the free smartphone app 'libra' and weigh yourself daily if it's not too much hassle.
This averages the daily figures to work out the trend.
Usefully it also displays calories over under - if you're losing weight, your deficit or the surplus for gaining.

There are lots of posts in the Fitbit Users group about using Fitbit + a HRM.

I'm maintaining using a Fitbit. As I said above, it's all about trial & error.

Most my exercise is step based right now because i'm race training and only end up doing maybe 15-20 minutes of strength training. I stopped logging the strength because at absolute most it might have only been 200 calories which can easily be negated by food logging inaccuracies.

My situation is a bit different than yours, since I'm still trying to lose, but last year, when I logged my hrm calories and ate them all back (while set up to lose 1 lb a week with mfp calorie goal) I maintained my weight. I'd be up or down 2-3 lbs but stayed pretty much that same for 6 months. I think this was because 1) I ate back allllllll the calories burned because running makes me HUNGRY. 2) because I took the hrm calories burned as face value. I'm sure you've heard of bmr and all that. You burn calories just being alive...yada. So for a more accurate calorie count, it might be a good idea to figure out how many calories you burn an hour naturally, and subtract that number for the same time period from your hrm amount and then log that number.

It can all be a bit maddening in the end. So just continue to log, monitor your weight and adjust your calories every couple of weeks.