Accutane can cause depression, but I heard a Doc comment that Acne can also cause depression. His perspective (and this is his specialty) was that you need to monitor how the person is feeling, but if the acne is bad enough to need accutane, then the trade offs are probably worth it (especially if the patient is young and a bit more sensative to how they look).
Dosage is weight dependent. In the UK, they prescribe less accutane for a longer period of time than in the US. Overall, they want you to consume the same amount.
If you are just trying to keep acne down while on gear, I know that many folks just use 20mg a day. That is different from a regular dose designed to get rid of acne for good.