I'm a math idiot but to try and learn more I've been playing with the Orbital calculations in KSP and made some basic calculators. I made one to work out the periapse from orbital period & apoapse. Calculator it's built in HTML5. It'll also tell you the what the circular orbit will be for a given orbital period by clicking the circle between the apoapse & periapse text boxes. Also be sure to click the "total seconds" button before the calculate button. Hope it's of help to someone. Ps only works for Kerbin at the moment