Hello. It sounds like you have a lot of good advice from your trainer already. As I am sure you already know as you sound pretty educated on this topic, weight ranges are wide specifically to accomodate the fact that people have dramatically different musculatures and frame sizes. Given that you already have a specific body fat % number from your trainer to work with, you have probably decided on your own what % body fat that you want to hit. I can tell you that if you want to get below 18% you have a very small window before you stop being able to get your periods (15%), so I would say IMO that 18% itself is already great maybe 17% at the lowest.
To answer your specific question about whether the HCG will work to get you where you want to be.. below 18% body fat as opposed to simply where your body believes it should weigh, you may want to consider doing further reading on Dr. Simmeons protocol. It all comes down to whether you truly have any abnormal fat deposits. For example, if you only have 10-15 pounds to lose but it seems to accumulate in a particular area or parts of the body resistent to lose, you may happen to have abnormal fat to lose. If on the other hand, you are very close to your ideal weight and generically have 10 lbs spread throughout your body that are actually normal fat stores, your goal to get the overall % down wouldn't be met with HCG per say.
I can tell you that I read a post on another HCG board from a gentleman. He lost a significant amount of weight going from a 42 to a 30 inch waist. Eventually his % body fat was too low and the HCG stopped working for him mid round descpite the fact that he could have in theory reduced his body fat % slightly overall, because all he had left were normal not abnormal fat stores. On the flip side, I came across a HCG medical clinic's website where a doctor tells a story of a patient who was a fashion model. She had been turned away by numerous doctors for HCG due to her weight being below the scale yet when the doctor looked at her body fat, she was actually obese from a body fat percentage range. She successfully used the HCG to lose the abnormal fat that she needed and then added some much needed muscle to her frame through weight training. She ended up two sizes smaller but at the same starting weight.
So it isn't an easy question for anyone to answer for you specifically, however, you may be able to figure it out yourself.