your 8 rep max will be about 75%
It will be lower if you haven't done 8 rep work in a while. Can we agree on that?
Or, to re-phrase it, it will still be about 75%, but you won't be able to perform it if you haven't done higher rep work in a while.
You'll be doing about 70% until you get into the groove of 8RM, and then you'll be up to 75%.
That's what I don't like about changing rep schemes every week. You could be lifting more on both your 5RM and 8RM days (and therefore give a bigger training effect) if the rep schemes weren't alternated a week apart. Think about it.