The "guideline" is 10 - 12 times your body weight in calories a day to satisfy basal metabolic funtions (things like cell division, cell respiration, breaking down of proteins into amino acids, etc). This does not include any activities you do (like lifting weights or using a stairmaster) nor does it include your lifestyle (active, sedentary, desk job, etc). If you are eating below your BMR you run the risk of putting your body under stress which will eventually lead to it shutting down to conserve fat stores - you will lose weight at first, but you will be losing more lean body weight than fat and eventually you'll be losing only LBM.

Fat grams comes into play because you've taken the carbs out of the scenario. You must satisfy your minimum protein requirement first (which is a minimum .6g of protein per lb of LBM and should not be less than 60g of protein for women). The remainder is made up of a few carbs (20 - 50g) and fat. Because fat is metabolically inert eating it in the absence of carbs will not cause it to be stored as fat. You cannot store fat in the absense of insulin. Glucose (carbs) causes a release of insulin. Fat will not have the effect on your body.

Think ratios or percentages and you'll do fine. If 5% of your intake is carbs, and 25-30% is protein that leaves you 60-70% for fat. I generally eat over 100g of fat a day and have lost 60 lbs in 20 weeks. I found eating more made me lose faster; eating less brought my loss down to 1-2 lbs a week. I'm not saying you will lose 3 lbs a week, but whatever your loss it will be more steady and sustained if you're eating enough.

People actually break stalls or plateaus by eating more; eating more fat. Because fat is 9 calories per gram it is easier to get the calories up there; 1 tbsp of butter has 120 calories and no carbs. The same holds for 1 tbsp of olive oil. Eating 300 - 400 calories extra of protein can be difficult; meat is very filling. Because ketosis decreases appetite eating more is often difficult. This is why we say "fat is your friend"

In the past powerful posts forum (off the daily low carb support forum) you'll find a thread called "Where's the fat" - it's a good read for information about BMR and eating enough.

HTH
Nat