In general they are pretty accurate. But at nearly 300 lbs you fall into the category of someone who is large enough to lose at a slightly higher rate than spark lets you set. Something in the range of 3 lbs per week for just a little bit, then down to 2.5 for a bit longer, then down to the 2 max that spark sets. Head over to Coach Dean's page to read about his experience and manually calculate www.sparkpeople.com/resource/calorie_calcu
with the slightly higher loss per week. As you hit 250, bring it down to 2.5 per week, then 2 as you get closer to 200 and down to 1 or less as you get close to where you want to maintain.