This is a 180day chart, covers the initial public release of the farming protocol ($1,551) all the way to today (current price is $145). That is a 90.65% decrease in value, or an average of about 5% decrease per day.
Back to school, you obviously skipped some math classes.
At 5% per day, after 180 days XCH would be worth $0.152
It was actually 1.2% decrease per day
It's actually a typo but let me more clear: on average, chia loses about
$7.81 in value over the 180 day period, which is
0.5% (half a percent) of $1,551.
Hope this clarifies.
Now, if you were to add on the percent loss (aka compound decay), then it would be
1.3% per day, so I guess your math is close enough.