Help with FSNav and PDMG 600/700

OK, just learning the FMC with the PDMG 737-600/700 I want to create my own flight plan using FSNavigator for Manchester EGCC to London Heathrow EGLL. I have done this and let FSNav create the waypoints for me. I also have the export module from FSNav to PMDG and have exported the flightplan out to PMDG. My question is how do I calculate the altitude that I want to fly the plane? Would you fly the 737 on a flight that distance at FL300? Is there a formula based on your climb speed and nM from Airport A to Airport B to give you your cruise altitude? And what about any of the other misc items that require inputting into the FMC? If someone could create a flight plan for the above the way it should so I know how to do it then that would be great! Once I've got the hang of it and can see the way that waypoints work etc then I guess I would be ok for future planning. Thanks