I don' t know if you can tell from how high the HCG levels are. A lot of people's are different: some can't take a pregnancy test and have it work because their HCG levels are low, but they are pregnant, while others at the same point in time in pregnancy can take a test and have it work: I think it depends on the person.

Usually you ovulate 2 weeks before your period. In this case just put your guestimate at the middle of the two dates. It will put you pretty close. When you go in for an ultra sound, they should be able to give you a pretty accurate due date/conception date.