14% is the "ermäßigter Beitragssatz"
+ 0.6% are the part for getting sick pay from the 43rd day of illness.
+ 1% is the Zusatzbeitrag, which differs across public health insurers
= 15.6% for public health insurance
If you opt to get money from the Krankenkasse from the 43rd day off sick, it's 15.6% public health insurance + 2.6% nursing insurance if you don't have a child. 2.35% if you do), in total 18.2%.
If you don't opt for that it would be 15% public health insurance + 2.6% nursing insurance if you don't have a child. 2.35% if you do), in total 17.6%.
However, public health insurance assumes for self-employed a minimum income (= profit) that rises each year (details in here) of:
30/40 * Bezugsgröße = 0.75 * 2,905€ (in 2016) = 2,178.75€
Let's put all details together, for the case that you don't opt to get sick pay from the 43rd day of illness, you would pay as a self-employed in Techniker Krankenkasse at least:
(14% + 1% Zusatzbeitrag + 2.6%) * (30/40 * Bezugsgröße) = 17.6% * 2,178.75€ = 383.46€ per month
Or you could just use Techniker's contribution calculator :