Wattage on a coil is to taste, this question gets asked a lot and there is no silver bullet answer other than to start low and gradually work your way up, get a harsh, scorched, or otherwise unbearable hit, back it down a couple notches. A lot of variables are at play, type of wire, exotic (which a clapton is), twisted, parallel, number of coils, resistance, diameter, horizontal, verticle, atomizing chamber size, airflow, nicotine level, pg/vg ratio, flavor, wattage, and finally the biggest factor, the end user (and everyone has different tastes).
Highlighted in red above I will give you a cautionary here, the Sony VTC5 is rated at 20amp Continuous Discharge Rate, and with the Kbox 200watt having a shutoff voltage per battery of 3.55volts, in series your max watts they can handle will be about 130 watts
130watts/7.1v (3.55X2)=18.3098 rough amps/0.9 (90% mod efficiency)=20.3443 final amps