Your battery life looks normal to me for the 8. I updated my 8 to iOS 11.4.1 and so far the battery life seems better when I'm not using the phone, the battery settings doesn't indicate the usage and standby time to be almost the same, despite not using the 8. Before iOS 11.4.1, my usage and standby time is almost the same even on days when I don't use it.
But I'll see over the next few days to see if it stays this way.