A different way to set up MFP goals - that WORKS!

Actually, I found that the math wasn't really necessary. Calculate your numbers as Olivia outlined (which you already did), and pay attention to a few numbers at the end of the day. Here's how I see your scenario:

First, you go to goals, and list your new calorie goal/day as 1620. Since you set your activity up as sedentary (as I did also, since I have a desk job), you need to enter your exercise for the day into MFP for this to work.

Now the few things to pay attention to: 1) You want to eat at or above your BMR every day, so maybe start with an "eating goal" of 1400/day. This is regardless of how much exercise you did or didn't do. Just memorize 1400, and eat at least that much.
2) Next number that could be important if you exercise a lot on some given day is shown on your daily summary as "=net". At the end of the day, after you have logged your exercise, you want that number to be ABOVE 1000. If it is not, keep eating until it is. A number below 1000 is a too high/not safe deficit.

Now here's the fun part for me: The green number on your daily summary at the end of the day gives you a really good idea of how many calories you "saved" toward weight loss for that day. A green number of 250 would translate to a weight loss of 1/2lb loss/week if you had that number every day. 500 would translate to 1 lb loss/week, etc. If you want to do a weekly avg, you could look at your weekly "calories remaining" and divide by 7.

I think the hardest mindset I had to break on this system is that the words "calories remaining" no longer mean what I can still eat that day, but rather what I have "saved" by not eating to my current maintenance. My deficits are smaller now, but with the built-in safety nets (eat at or a bit above your BMR, and don't let "=net" go below 1000), I no longer worry about whether I am eating too little, exercising too much, etc.

This is what I've done, it's nice and simple.