This is very individual, and depends on what your goals are as well as what your definition of "low carb" really is.
First, figure out how much protein you want to eat in terms of grams. There are a number of calculators out there to help you figure it out, but for general principles, starting somewhere in the range of 1 gram per pound of ideal body weight might get you in the right neighborhood.
Then, determine how many carbs you want to eat in terms of grams. Doing Atkins induction? Less than 20 carbs? Somewhere in the OWL ladder? This could be anywhere from biological zero carbs up to 150! Some people consider low carb to be anything lower than what is recommended by the Standard American Diet.
My last known good calories that put *me* into optimal ketosis was like:
1357 calories, 114 gm fat, 63 gm protein, 25 gm carb
This works out to percentages of:
76% fat, 19% protein and 7% carb
Since MFP doesn't allow you to do anything but 5% increments, my MFP percentages were:
80% fat, 20% protein 10% carb

I have calculations for determining lean body mass and calculating macro percentages under the "Calculations" section on my blog's first page here: http://www.myfitnesspal.com/blog/PatchEFog

Thank you so much!!! I did not know that spices have that many carbs. I thought they were obsolete from carbs!!! I started spicing up my meals last week and only lost six ounces so I am wondering how many carbs that I really took in. Wow I learned something new today.

Thanks again.

You're welcome.
There are carbs in spices. One needs to be especially careful of spice blends since carby ingredients might be added like sugar or maltodextrin for bulk. Because of the labeling guidelines, they don't have to divulge partial carb counts per servings, and of course, they set their own serving sizes.
Here is a lookup for some more common spices:
http://calorielab.com/foods/herbs-and-spices/49

Also, brewed regular coffee and teas are generally half a carb per cup as well.

It's a pity that MFP rounds their values. I'd found that it could be something in the range of 7 carbs different (less) than what I calculate on my personal spreadsheet for the foods I eat.

I like this calculator: http://keto-calculator.ankerl.com/