I think your suggested rate calculation (15% above salary divided by 2080) undervalues the rate. Your overhead as an independent includes additional employment taxes, retirement contributions, hardware, software, utilities, and space. In addition, you are taking on risk that is built in to your company-provided salary.
I believe you should strive for 100% above your salary. If you make $100K, your goal should be $200/hour, especially if the engagements are small/ side work. You probably should not settle for less than 50% above, as you are setting a rate that you will use if your side work becomes full-time (which it very well may).
Don't undervalue yourself. And don't count on going in at a low rate and raising it later. Set a rate that reflects your true value.
Remember, companies pay $250+/hour to large firms for specialized consultants whose take home pay might be $100K. That's who you're competing against.