Ok, basically your BMR is your Basal Metabolic Rate which is the MINIMUM amount of calories per day that your body needs to sustain itself at rest. Any less than that and your body shut itself down as people have mentioned before, your metabolism will run and hide leaving you feeling drained. And then, when you increase calories, your propensity for storing fat will increase... BOOM, yoyo diet/crash diet effect in action...

The was a formula developed by Harris Benedict to predict your BMR...
Women: BMR = 655 + (9.6 X wt in kg) + (1.8 X ht in cm) - (4.7 X age in yrs)

Heehee - were you good at algebra at school (don't you wonder how these people worked this out!)!

So now you've got this number but your body's total calorie needs are never just the basic BMR level... Use a multiplier based on your lifestyle to calculate how much you'd actually need to consume (1.2 for relatively inactive, 1.4 for reasonably active (say 3 sessions of exercise per week), 1.5 or 1.6 for more active (say 3 or 4 sessions per week plus a reasonably active job) and even double for those who are extremely active). Create a calorie deficit by giving yourself a shortfall of 500 calories per day without going below your basic BMR... and bingo! Spread those calories over 6 meals a day and your body will never feel deprived...

What exactly are you trying to do to your thighs?! Do you want definition or lose fat? Unfortunately you can't spot reduce fat on the body If you want more definition then i'd recommend leg extensions if you have access to a gym the leg extension machine is awesome for toning your quads and leg curls are great for your hammys. If you don't have access to a gym then squats and lunges are great if you've been doing them for a while you could try adding some weight (held or on shoulders) or walking lunges or introduce some instibility ie put one leg up on a bench behind you and do one legged squats. This forces you to "switch on " your core working your abs as well (but only if you're confident doing them properly).

HTH