which - identifying the correct noun when dealing with a possessive

I can't figure out the noun which is referring to in the sentence below, percentage or users, and thus don't know if the verb "amount to" should be plural or singular.

50 percent of internet users (which amount to 60 percent of the world population) say that...

To my ear, it should be amounts to as the verb is referring to the percentage, which is uncountable. However, in the original text, it was singular, and could easily be referring to internet users, could it not? In which case, the above sentence would be correct as a singular.

Thoughts?