I've been struggling to parse the following sentence from the UK petition to implement EU Referendum Rules triggering a 2nd EU Referendum.
We the undersigned call upon HM Government to implement a rule that if the remain or leave vote is less than 60% based a turnout less than 75% there should be another referendum.
I'm puzzled at the phrase "less than 60% based a turnout less than 75%". If it were something like "less than 60% based on a turnout of less than 75%", I wouldn't have any trouble, but apparently it isn't. Especially the usage of "based" like this, without any preposition, seems quite unfamiliar.
How should I parse and understand the sentence?