Yes! I know how to program potentiometers all too well
We had four of them on our robot last year, with pre-defined postions involving all four of them. I had many bugs in that code ... but it sure was fun getting it to work since that was the first time I'd ever used pbasic before.
Now, to your question. Are you talking about the torque motor? I know that doesn't allow full rotation, so is that "the thing that allows only 90 degree turns." Or do you want to use a pot and limit something to only 90 degree turns? Like Mr. Johnson said, please clarify your question and we'll all be glad to help.
Stephen