The "sensitive" heart is a bit concerning. If you have a heart condition, you should avoid all stimulant drugs. Otherwise, Ritalin and Adderall are different, two different mechanisms of action, two different classes of drugs. For a healthy individual with no stimulant tolerance, I think 20 mg Adderall XR would be about right for your purposes. A little less might be ideal, but if you don't have a heart condition, you will be fine. The only thing is though, and this applies with most stimulant drugs, is that a higher dose can lead to distraction, and a lower dose prove more effective for purposes of studying, test-taking, cleaning house etc. I don't think 20 mg is too much if it is XR, though.