Adderall newbie, Is this safe....?

Okay so, last week i took 20mg dosages of adderall like 3 times and felt great. It really helped me get my work done. Today I have to write a paper and study for a midterm. I got a 30mg pill. I took 3/4's of it at 1:30 and im feeling good now. But I wanna study for along time. What would be the best time to take the other 1/4 of the pill. Will I be overdosing? Is this safe? Please no comments about how drugs aren't okay if there not prescribed to you. This really helps me get work done, and I need to do good this semester so please no bad comments.