Well, with the data you can calculate it more exactly, 12m is short enough, and cat6a is good, with 12W you lose only 0.4 V and 0.2 W @24V
http://poe-world.com/Calculator/
Voltage at source end = 24 Volts
Current = 0.5 Amps delivering 11.8 watts remotely
Distance 39.36 (ft)
Resistance 0.799008 (Ohms)
2 Pair
Line Loss 0.2 Watts per device
Voltage at remote end 23.6 Volts
Current 0.5 Amps
watts per device (from injector) is 12 watts
With 28 V/14W it is of course a little better :
Voltage at source end = 28 Volts
Current = 0.5 Amps delivering 13.8 watts remotely
Distance 39.36 (ft)
Resistance 0.799008 (Ohms)
2 Pair
Line Loss 0.2 Watts per device
Voltage at remote end 27.60 Volts
Current 0.5 Amps
watts per device (from injector) is 14 watts