The Breslow No. - How accurate is it?

I had what was to all appearances only a very small lump removed five months ago from my forehead. The pathology returned a melanoma which was quite a shock. The path report gave it a Breslow No. 7, which I came to understand is BIG and therefore the prognosis bad. I subsequently had a PET scan which was clean and then surgery to widen the excision margins and removal of a couple of facial lymph nodes. Fortunately, the pathology was negative. I was then passed to the oncologist and she told me that I wasn't a candidate for chemo - I understood because all the subsequent pathology and the PET scan were clear - and she recommended that I join an experimental vaccination treatment she is running, using melanoma cells, as a treatment designed to prevent recurrence, but because it is experimental there is no guarantee of its efficiacy or success. I signed up and am now in the treatment course. I have seen the oncologist a couple of times since starting and I must say that when I look in her eyes I get the feeling that she thinks I am a lost cause. Of course it might just be my imagination running wild... and I see the reason being the size of the Breslow No. Nevertheless, I continue as ever, I work and golf as usual, do a lot of work related travel internationally and have had no side effects from the treatment. Like all cancer patients, some more than others I have noticed, I do have moments when I get worried. When this started five months ago I thought I would be dead by the end of the year and started to prepare my family and friends, but it is now February and so far so good and I feel just as I ever felt. Nobody seems to be expecting me to fall off my perch any time soon. The problem is that I have at the back of my mind all the time the big Breslow No. and the fact that it was so big, which translates as a bad prognosis.
Can anybody provide some information about the Breslow No. and what is says about the prognosis. Do people with big Breslow's survive? I am afraid to ask the oncologist because I may not like her answer.