I believe that in cases of percentage grades, each class grade is calculated into a letter grade and they are added up to form a cumulative GPA. 85%+ A, 80-84% A-, etc. LSAC should have a chart somewhere that shows you how to convert each one of your grades and then you can put them into a GPA calculator and it should give you your overall GPA.
The good news is you have a valid reason to write an addendum helping to explain your GPA.
The bad news:
"but LSDAS knows canadian GPA's are different, and can't be converted using that scale. As far as I know, you need not worry."
This is incorrect. While SOME schools might understand, or be made to understand by your addendum, that your Canadian grades in this case have been calculated differently, they must none-the-less report your LSAC GPA for statistical purposes (U.S News rankings) which will make it difficult to be admitted to a top-tier school.
I would contact LSAC and see if there is anyone there who can help you with the specifics of your case. This isn't to say you're completely screwed. A strong LSAT, softs and a good addendum can still give you a fighting chance.