Figuring out the moment when correct information started flowing in can be most easily done by looking at the frontend. Navigate to Monitoring | Latest data, click on History for that item, and then select Values or 500 latest values. Look around the time you changed the item multiplier plus a minute or so, and check for a notable change in the scale. While it might be hard to pinpoint the exact interval between two checks (network traffic can easily fluctuate over eight times in value between two checks), there should be a pretty constant increase in values. Look at the times to the left of the values and choose a moment between the first good value and the last bad value.