Demodectic mange CAN often be treated with ivermectin but you need to use this only under a vet's care. Your vet can tell you the dosge information, which varies as the treatment continues.
Your dosage will be configured based on your dog's weight and the length of treatment that is needed. If you overdose your dog with ivermectin, you can cause blindness from which he may not recover.
So - to answer this question- ivermectin can be used to treat demodectic mange but only a vet's instructions.
Let me know if you still have questions.