We usually use 1000 Ohms, 5 W, resistors but if you want to reach 250 mA you need a lower resistor. I suggest you a 470 Ohms 5 W resistor, you can buy it in any ordinary electronic shop; with it it will be very easy to reach 250mA in Muscle mode.
You can ask for details to Eng. Kingnara or Eng. Jakkujan in Service dpt.