Folks who have lost 50+ pounds

I have really never eaten fewer than about 1500 calories a day (except for my first month or two, when I think my base calories were set at 1420).

As I've gotten closer to my goal weight, my rate of loss has slowed down, but I am still losing (with about 8 lbs to go, unless I drop my goal further), and the scale is heading in the right direction. During the times when I felt like I might be plateauing, I realized that I was slacking a little and not logging my intake as accurately as I had been, or that my activity level had decreased while my intake stayed the same. Once I buckled down again and was honest with myself, things started moving again. I use a food scale to weigh the majority of the food I prepare at home.

I think it's important to do some research so that you're well aware of your TDEE and your body's energy needs. Don't just pick a random calorie goal. Know your approximate maintenance level and what kind of calorie deficit you need to create to lose weight in a healthy and sustainable way. And as mentioned above, it's important to recalculate your body's calorie needs as you lose weight (generally every 10 pounds or so), because the less you weigh, the fewer calories your body needs to maintain itself.

I have never eaten so little. I think at my lowest goal, I was netting 1400 calories. But I was also exercising 500 calories a day on average, so I was eating more like 1900.

There have been days when my net has been below 1200. Heck, there were 2 or 3 days when my net calories were negative (a 9-hour bike ride in the mountains will do that). But I compensated for those on other days in the week.

Bajoyba's advice is spot-on: know your BMR and total energy needs (TDEE), and set your goal based on that. You can use online calculators to estimate those figures, but if you log accurately, your actual progress is the best way to figure out TDEE.

How would you calculate it based on actual progress?

If you're accurately logging calories, then you know how much you are eating every day.

If you're weighing yourself regularly (at the same time of the day), you know approximately how much weight you've lost over a given period. (It can be more accurate to use an exponentially smoothed weighted average - see John Walker, The Hacker's Diet, http://www.fourmilab.ch/hackdiet/e4/)

With those figures, you can calculate your actual TDEE as follows:

Add up the number of calories you ate in the period. To that number, add 3500 times the weight you have lost in pounds. That will be your Total Energy Expenditure for the period. Divide by the number of days in the period to get TDEE.

Example: My weight on March 31 was 163.18. On April 30, it was 162.11, so I lost 1.07 lb. in April. I logged 63,203 calories in April. So my total energy expenditure for April was 63203 + 1.07 * 3500 = 69948. April has 30 days, so my TDEE was 69948/30 = 2231.6. Of course that's a daily average. I logged 14424 exercise calories, using fairly accurate HRMs, for an average 480.8 exercise calories a day, so my NEAT (Non-Exercise Activity Thermogenesis) was about 1750.8.

According to a few TDEE calculators, my NEAT should be around 1950. That suggests one of two things: (1) I've been missing up to 200 calories a day, which is entirely possible (see http://www.ncbi.nlm.nih.gov/pubmed/12396160), or (2) I'm experiencing adaptive thermogenesis--but 200 calories a day is a pretty big AT effect. It's probably a mixture of both.

It doesn't really matter, though, as long as I am consistent in logging, and use my results to adjust how much I eat.