The change on the first day is 0-0.25 =-0.25, which is 0.25. Just take a positive number. 0 is the normal water level.

The reference water level of the next day is -0.25, so the change of the next day is -0.25 to -0.64, which is equal to 0.39.

The third day is -0.64 to -0.54, which is 0. 1.

And so on, 0.67 on the fourth day.

Day 5 0.26

Day 6 0.08

Seventh day

The change of total water level is 1.79.

If you want to use the formula, you should omit braces after [0-0.25] +{0.25-0.64} to indicate the absolute value, and then the change from the third day to the fourth day is negative, and the positive value should be based on the normal water level, that is, {0-0.54}+(0+0.2 1).