Liquid load inside a tank

I am trying to analyse a tank which will be supported by four supports on its sides and bottom of the tank will be about a feet from the ground.

I found one tutorial on Analysis of a Fuel Storage Tank.
In this tutorial, pressure value is defined as 1 and Y as 0.029 but it has not been mentioned how to calculate these values for different cases.
Can somebody please tell me how these values are calculated.

For my case, I am using a liquid of 16 PPG and tank is 3 feet high.

Thanks
Mandy