you can use 2 nos of 0.1 ohm resistors( to make it 0.5 ohm) and for 15 amps it would develop .75 V
the wattage needed is 15*15*.05=11.25. So using 10 watt 0.1 ohms 2 nos would be safe enough.
Finally whether it would limit at 15 amps or 10 amps , or higher depends on the voltage needed for the transistor (opto isolator) to switch.
Later if needed, you may use a potentiometer of say 100ohms across the current sense and feed only a part of the drop to the transistor and it would help effective control of the current.
How ever i have my doubts on ETD44 core -- whether it could deliver 15*16= 240 watts ( say 300 watts including the system efficiency).
Try out better core with higher capacity.