When trying to calculate how much I should eat for weight reduction what should the base calories be based on? My current weight or my goal weight? I'm 5'8" at 240lbs wanting to drop to 180lbs or less if at all possible.
Base it on what you're currently eating, which you know is probably too much. So just log what you're currently eating, adjust the macronutrient ratios, and then adjust the total intake downward.
Log your diet for a week. Don't do anything special, just log what you'd normally eat.
Note the balance of carbs/proteins/fats and aim to bring it more in line with an accepted ratio. Here are some options:
Or, a paleo-type diet:
Once you've established a well-balanced diet, you can start fine-tuning the amounts. Reduce your intake slightly and increase your activity level to promote fat (not necessarily weight) loss. As long as you're noticing improvement at your chosen activity (weightlifting, running, swimming, etc) and fat loss, you're moving in the right direction.
What you need to know is how much energy in calories you are expending every day. This equation is:
Basal Metabolic Rate (BMR) + daily living activities + exercise activity = total caloric need.
BMR is a calculated guess, unless you are lucky enough that you can have it directly measured using some form of calorimetry. Some of the calculation equations are the Harris-Benedict, Harris-Benedict revised, Catch-McCardle and Cunningham. You don't need to know these formulas, there are tons of them premade into calculators on the internet (The two Harris-Benedict formulae are the most prevalent). BMR is basically an estimation of how many calories it takes you to simply lay there and breathe in and out for 24 hours.
Daily living includes normal activity such as work, shopping, house cleaning, etc. Exercise is, well, exercise.
There are tables all over that have estimates for those activities as well.
Now for the weight loss. This is going to be variable. As is pointed out in the comments, the 3500 calories per pound is bad mathematics misapplied, and started with an observation in 1959 by a Dr. Max Wishnofsky. Calculate your caloric needs, add in your activity cost, and eat less than that each day and you will lose weight. The rate at which you do will vary with your own metabolism, activity, types of food, etc.
I would start by calculating what your needs are, then keep a food log for 3-5 days, writing down EVERYTHING that you eat. Weigh your portions, and get the best count you can for calories. That should give you a starting point for your weight loss.
If this is the tactic you're taking, I highly recommend the Hacker's Diet by John Walker (available as a free website/ebook). The theory is pretty simple: track your weight and calories daily, and reduce calories when your weight trend line is trending up, increase them when it is trending down more than you'd like. He's created a simple online tool to track your weight and generate nice trend graphs. Feedback based on the weight trend automatically takes into account reductions in metabolism, exercise, etc. The key point is not to take any weight measurement seriously by itself: keep calories/diet reasonably consistent and look at the longer term trend.
That said, while I love Walker's weight tracking tool, I've never found calorie tracking very tenable. So tedious and difficult to keep up with. I much prefer adjusting the foods I eat until I find a diet that leads to weight loss over time. The trend line is still useful for determining when to change things or when changes don't work, but calorie counting is not for me.
I agree with JohnP and Sancho's posts, I just wanted to add that I'd specifically found that using an calorie tracking app on my smartphone made calorie counting and recording my meals practical.
I've been having luck using a calorie counting app on my smartphone. I'm using the Livestrong app / website. I don't know that its better or worse than other calorie counting apps / websites out there, but it seems to have a decent user interface. It has daily caloric goals based on your current body weight, and lets you log the food you eat and the amount of calories you burn to see how close you are to your daily goals.
I've been using it since February, and I've had a fairly successful run of weight loss. I'm 6'4", 185lbs, down from 209... so I know I'm not looking at the uphill battle you were, but I've found that having a basic record keeping system (like the iphone app) seems to have been much more helpful at keeping me conscious of how much I've eaten and exercised. Its really changed my relationship to my meals. With better record keeping, I'm able to see when I tend to over eat, how I need to adjust my diet, etc.