You are usually given a timeframe in which to take a test and so this will more than likely govern when is the correct time. Never leave it to the night before as grad rec will not be on hand to help if a link doesn't work etc and, sadly, if you cannot take the test at a late hour for something like this, your application may well be nullified.
@Jessica Booker has written so so extensively on the subject of the WG and why these pass marks don't mean a lot. Yes, when practising they are a great gauge to see how you are doing, but law firms have their own benchmarks and 'pass' marks based on the study group they are assessing you against. It is pretty complex really and not a one size fits all situation so 70% could be great for one firm but poor for another. I would say though that for the testing stage, you sound like you are doing well and 70--80% are good scores on the face of it.
