Your 76.3GB partition is fine, leave it be now that you have it
HDD makers report their sizes as if the byte was 1000
not 1024. It's a slick little way that allows them to slightly inflate the size in ad's. You are correct that 76.3 is what you can get out of the drive since a block is 1024.
If a block was 1000, the drive would be 80gb.
Unless your using graphics intensive (and I don't mean gameplay, I mean vector works) programs, your fine with one partition. Same goes unless your doing data intensivve work. In some cases of constant high load, having a single large partition creates a bottle neck from seek times. I can't see that applying to you though.