MyFitnessPal is free and OK. Put in your weight, say you want to keep it constant, it will give you a number of calories. What I will say is that it assumes a 'Standard American Diet' level of carbs, which is not what you want, obviously, but you can adjust that manually to meet your personal carb aims and then adjust your fat amount/proportion accordingly. The other thing I have found is that it assumes that 2250 cal of low carb/high fat has the same effect on body weight as 2250 cal of Standard American Diet, which certainly isn't my experience. So it keeps telling me that I am going to put on half a stone in a month, eating at the rate I do, which I definitely don't! Like you, I'm trying to stay stable or even put on a few pounds.