Extra calories for exercising

Depends on your goals. If its to lose weight then no. If you want to maintain or gain then usually yes you should try to stick close. I burn a lot each day and almost never eat enough to get to my calorie goal. Hope this helps!

This is not terribly good advice. MFP assumes that you are eating back all exercise calories, and the closer to your total goal, the better. Undereating will promote weight loss, but it will not promote fat loss. At small deficits, your body will tend to use fat reserves as they are readily available and easily obtainable. As the deficits get larger, your body will begin to hoard the fat deposits against a continuation of the large deficit and will start to burn lean muscle tissue in its place. The longer this trend continues, the more likely it is that your body will shift the ratio away from burning mostly fat. To maximize fat loss, you should eat back all exercise calories. Your other option is the TDEE method, as explained in the link below.

http://www.myfitnesspal.com/topics/show/975025-in-place-of-a-road-map-short-n-sweet?hl=short+and+sweet&page=10#posts-15694678

20 pounds is an ambitious goal, but theoretically possible.

http://www.myfitnesspal.com/topics/show/975025-in-place-of-a-road-map-short-n-sweet?hl=short+and+sweet&page=10#posts-15689310

The above will be a fairly good guide to determining how many calories you should be eating.