Spartacus, snapshot manager IS installed by default and it's not unusual to see a person creating a snapshot on day one (as I've said above) - so once again, please check your snapshot manager.
If you try to create your C: and E: to 80GB each, and D: to 800, you will use 960GB of capacity. Your screenshot shows you have 925GB of capacity, surely at this point - "would you recommend I overallocate again" is not really the question?