Canadian applying to US - GPA

Hello, I have tried calculating my GPA numerous times, on my own, using different websites...I seem to be get great degree of variation. As such, can someone tell me how to most efficiently calculate my GPA? I am a Canadian student, so I converted my percent grades to letters based on i.e., A+ (90's+), A- (80-84), etc...and from there I convert a grade of A+ to a 4.0...A- to an 3.7...etc...is this the right method? slightly confused here. Any help is appreciated! thanks

p.s. we have a...

Canadian applying to US - GPA