Truncated Data Analysis - Standard Deviation

#1
It has been too long since I have utilized statistics, so looking for a little simple instructions. Need to verify if my approach is at all correct?

I have a problem where we have recorded the volume liquid entering a tank every day for 16 months. I would like to use statistical analysis to predict how much liquid we will get on any given day to ensure our vessel is designed adequately.

From my searches, I get the impression that this would be Truncated Normal Distribution ? (as opposed to folded-normal distribution) ... not sure i really understand the difference.

The data is more frequent the lower the gallons. See image


Can I still use excel to determine Standard Deviation?

What I have done so far:
Eliminated the days where zero (0) was recorded as the process was not running those days. Calculated Mean simple enough (1260 gallons)
used the STDEV.P function in excel to calculate Std Dev = 1838 gallons

Now looking to know how large to design the process ...
Mean = 50% of the liquids would be 1260 gallons or less
1 Sigma (84.1%) would be 3098 gallons or less
2 sigma (97.7 %) would be 4935 gallons or less
3 sigma (99.8%) would be 6773 gallons or less

Standard vessel design is for 6,000 gallons ... How do I calculate the percentage of daily liquids this would be? What percentage of gallons received would be less than 6,000 gallons?