W2 vs 1099 Billing Rate

Is there a general formula for determining a 1099 billing rate should I choose to change my employment status from an employee to a contractor?

 

If I were to be a contractor who is then self-employed, is there a general rule of thumb to determine a billing rate? If I am currently a $50/hour x 40 hours / week, what would then be a typical billing rate to anticipate the additional self-employment related taxes?