Mark, does she only have coley and biscuits, i.e. no wet cat food? If so, I would be dubious about increasing her coley and decreasing biscuits as fish doesn't have high levels of taurine in it, so it might not be a balanced enough diet if she gets less complete food. If she has a sedentary lifestyle, that needs looking at too, you can feed her as little as you like, but if she isn't exercising as well, it wont work.