ELI5: How do you convert Australian uni grades into an American GPA?

Probably gave it away in the title, but I'm an Australian student with ambitions to go to grad school in the US.

My (limited and probably wrong) knowledge is:

• 80% or higher: A (4.0 GPA)
• 75-80%: A- (3.75)
• 70-74% = B+ (3.5)
• 65-69% = B (3.25)
• 60-64% = B- (3)
• 55-59% = C+ (2.75)
• 55-58% = C (2.5)
• 50-54% = C- (last pass grade?) (2.25)
• Less than 50% = D (1)
• Absent = F (0)

Working from here, if I hypothetically had a 75% Australian overall average would I have an A- (3.75) GPA?

Or would it be calculated on a class-by-class basis (Suppose I did two classes: If I got a 4.0 with 90% in one subject and a 3.0 in another with 64%, would my GPA be 3.5 - and therefore a B+ by grading - or averaged to 154/2 - 77% and upgraded to a 3.75 A-?)

This is all very confusing. All replies will have truly earned their upvote.

One last thing: is your declared major given extra weighting in your GPA at the expense of your other electives?