The Social Security Administration will happily forecast your future monthly retirement check. Trouble is, it’s often off the mark. Understanding the sometimes-flawed assumptions underlying the estimate can help you make smarter decisions about when to claim your benefit. First, of course, you should know how to access those estimates. You can find yours online by creating a “My Social Security” account at the Social Security Administration’s site, or you can call 800-772-1213 to request a paper version. (The agency automatically sends paper copies to people 60 and over if they haven’t yet started benefits or created an online account.) Social Security projects how much you’ll receive if you start benefits at the earliest age, 62, as well as what you’ll get if you start instead at your full retirement age — currently 66 and rising to 67 for people born in 1960 or later — or at 70, when benefits max out. When you apply for benefits, Social Security uses your 35 highest-earning years to calculate your check. Each of these years is “indexed,” or adjusted to reflect wage and price inflation over time. The dollar amount you earned in 1995, for instance, would be roughly doubled to r...