You left out the time required to market yourself and look for new gigs, the allowance for empty space in your schedule (it's hard to manage a pipeline even in

You left out the time required to market yourself and look for new gigs, the allowance for empty space in your schedule (it's hard to manage a pipeline even in a factory where you have insane degree of control; don't expect perfect scheduling as a consultant) and a safety margin to cover the clients who don't pay thir bills. That said, "double" is a rule of thumb. Physicist law states that 1.68 is equal to two for rule-of-thumb purposes. ;)