I would go by this:
So, yes, it looks like 87+ is an A.
If all your classes have the same number of units, then yes, you can just do what you did. But I highly doubt # of units/credits are the same for every class.
If your units aren't the same, you need to do it by individual courses, since # of units (and consequently unit weights) differ.
Count up the number of units you got in each percentage bin, and use this (
Cumulative GPA Calculator (UC Berkeley) - GradeCalc.info) to calculate your GPA.
For example, if you freshman year looked something like this:
Chem 1 (4): 87
Bio 1 (4): 92
Theoretical Particle Physics (5): 77
Basket Weaving (3): 85
Put in 8 units at A, 5 units at B+, and 3 units at A-.
Do that for all the courses you're putting in AMCAS, and that's your AMCAS cGPA/sGPA.