I'm trying to find a key point of difference, and I think I have found it: Let's assume the following is true (which I believe is a requirement for subjective value theory) "there is no objective purpose in the world".
Therefore statements like "I shouldn't eat ice-cream three times a day, even if I do so" can't be evaluated objectively as either true or false, because there is no way to determine what it is we should or should not be doing. A doctor may say it's true, as his job is to maximise you overall health. However for you there is more to life then just being healthy, and enjoying that ice-cream may be one of them.
Statements like "eating ice cream three times a day will cause you [insert health problem here]" can be objectively true, but you see it makes no comment on what you should and should not do.
Assuming the premise is true do you agree with the conclusion? If you do fine, then you must disagree with the premise, can you explain why? Or is it the third case, you agree with the premise, but disagree with the conclusion?