Use the normal gradient of water which is 0.433 psi/ft or 8.33 pounds per gallon. Mud weights are typically in pounds per gallon in the range of 8.33 ppg to 18.2 ppg which is usually the maximum anywhere in the world. Deep, geopressure wells in the Gulf of Mexico or elsewhere in the world.

As a common drilling practice, normal pressure gradient used is 0.465 psi/ft around the globe. 0.433 psi/ft is the pressure gradient of the fresh water (equal to 8.3 ppg density), whereas 0.465 pst/ft is the normal gradient for formation water, which has higher Chloride content (salty) in it compared to the fresh water.

Any pressure gradient higher than 0.465 psi/ft will be classified as abnormal pressure (mostly due to overburden pressure - and could lead to a "kick problem" during drilling), whereas the gradient lower than 0.465 pst/ft is called subnormal pressure (which could lead to "lost circulation" problems).

Normal formation porepressure is a formation pressure that equals to hydrostatic pressure of a column of formation water at the same depth of the formation.

Normal formation porepressure is highly dependent on formation water salinity in the area being drilled. Ranges of normal formation porepressure is from 0.433 psi/ft for fresh water to 0.465 psi/ft for high water salinity (100,000 ppm). During drilling normal formation porepressure no risk expected, drilling by freshwater or saline water is very enough, but drilling through overpressured formaton you have to consider the mud wight to overcome the overpressure of the formation being drilled, overwise will encounter what so called a kick.

https://www.glossary.oilfield.slb.com/Terms/n/normal_pressure.aspx