It is common to lose some during the day, but your loss seems a little high for me, but since you are still fighting something (hence the 2 ppm loss at night), that may explain it. What is your CC level?
If you can, try to not let the pol drop below you minimum shock level during the whole shocking process. This may mean you will need to test in the early afternoon. If you can not get there, and if you can not get someone else to do it, maybe attempt to add enough so when you come home you are not below 18 ppm (using the less conservative poolcalculator recommendations for the 18 ppm).