sorry that this reply took so long, was swamped with work and needed some time to familiarize with the chip.
i did some testing. first with a 22k Ohm resistor (had no closer value to 2k) theoretically letting 17,7 mA through and second with a 470 Ohm resistor one letting potentially 83 mA trough.
then measured Volts and Ampere
for a 10 cm strip (3 leds) i get between led strip anode an red cathode:
1) 11,68V (directly attached to the anode and the cathode) 18,9mA
2) 12,05V (same measuring as above) 18,24 mA
Those numbers don't look right. Normally more volts = more amps.
so am i right that only ((12,30 - 12,05) = 0,25V goes into the tlc chip which by my calculations would mean i am on the safe side for power dissipation?
my calculation is based on: 20 mA (into the chip) 0,25 V(into the chip) and 12 outputs used (4 led strips per chip)
That number, OTOH, does sound about right. The strips are designed for 12V, your power supply is putting out 12.3V... 0.25 is pretty close to 0.3.
0.25V for the chip to dissipate is plenty of margin. You should be fine connecting a string of 15 LEDs to each pin (ie. 5 groups of 3 LEDs, 20mA per group = 100mA).