You should aim at 100%, except that if you get a stupid question wrong that doesn't count.
So, in Air Law, if you can't remember the date some piece of international legislation was passed, who cares. It doesn't make any difference to your chances of a mid-air through not knowing the rules; but you should go for 100% on questions about right of way etc.
Or if you can't remember whether you're allowed to go flying with 2km visibility or 1.5km, who cares. In real life if it's less than 5km you aren't going to take off anyway, so the answer is irrelevant.
Similarly, in Met, if you can't remember the details of the theory of how a thunderstorm works, that doesn't count either. If it's a thunderstorm you won't be flying, so again who cares.