In my experience, when you get that message constantly, then you should probably -up- your target calories for the week to something that's about the average of what you've been working out for. I don't own an HRM either, so any numbers are better than guesswork for me.
I noticed a couple when I did adjust according to the past few weeks' calorie burn. For starters, your nutrition page will change. The handy mechanism behind the trackers will see that you'll be burning more calories, and will adjust your nutrition tracker accordingly. Secondly, when I adjusted mine to the calories I'd -been- burning, I noticed that I was barely hitting that mark any more. I wasn't as low as I've previously been, but I wasn't capping out anymore either. I was somewhere in between.
For reference, I lose about a pound a week, and am lazy so I don't necessarily exercise every day.
"A journey of a thousand miles must begin with a single step." - Chinese Proverb
Short-Term Goal: 170
Mid-term Goal: 150
Ideal Weight: 120-140
| current weight: 181.0