I have the same question. Did you find a solution?
I did find that using "Delivered (Week)" and then filtering to "Current week" produces the results I need, though it may not meet your criteria for most recent 8 days.
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There isn't an 8 day setting in the Setup tab of the report itself, but you could set it to 30 days and then use a Was Sent Email filter in your report's smart list.
This filter will allow you to say Was Sent Email > Is Any and Date of Activity > in past > 8 days.
That should filter out anyone that didn't receive an email in the last 8 days, so you're cutting the 30 days down to 8.