There is actually a solo course that you can take. It will help you understand all of the considerations including gas management, not just rock bottom limits or turn around times; I believe at a minimum you should use the rule of thirds. I have done it and if you think about it as an instructor not only are you often solo diving, with new students you have double the responsibility to ensure all is correct before you or one of your students end up in bubble trouble.
I will also say that there are places like Cooper River where you are black water diving and even if you have a buddy right next to you, you likely cannot see them . . . on a good day you may see their light. Rescue diving in some of the places here in the county is the same way as the vis is in some a few inches. While this is different from Cooper River, there at least you have a recreational objective, all require the same safety considerations, none to be taken lightly.
to find out more about Solo diving visit SDI, the agency that offers a solo course. http://www.tdisdi.com/index.php?did=44&site=1 Need an instructor, Contact www.BrunswickScuba.com