Difficult, isn't it. With my boy, even though I was aware of the risk of pancreatitis, I didn't worry too much about feeding him a very low fat diet. I just stuck to a reasonably low fat diet and avoided high-fat treats etc. We didn't have any issues, but each dog is different. If you feel that she's losing fat as well as muscle, you could try feeding her more to see if it helps, as it could be that she isn't getting enough calories from the low-fat food. Just watch that she doesn't start getting too much fat over her ribs, otherwise you'll end up with a fat dog with weak muscles, which is the worst of both worlds! I too was told by the specialist that dogs usually put on weight with pred, but it certainly wasn't true for us for the first few months. After that, he did start to get fat, but still with weak muscles. Her weight loss doesn't seem too extreme at the moment.
You don't want to be in too much of a hurry to reduce the pred because if you do she might relapse. This happened to our boy. There are lots of different protocols for reducing pred, but I have generally followed Professor Day's protocol, which is here just for your information: http://cimda.co.uk/smf/index.php?topic=368.0 If you add in another drug then you may be able to reduce the pred faster. I have no experience with this as we managed our boy just on pred. It's like walking a tightrope at times, weighing up the response of the disease to the drug against the side effects the dog is experiencing.