Calculating the hydrostatic pressure of a liquid column is straight-forward. In case you need a refresher, the equation is:
P-hyd [psi] = 0.052 * (Fluid Density [ppg]) * (Fluid Depth [ft])
The above equation calculates the exact hydrostatic pressure of a liquid column. But what about for a gas column? The answer is far less straight-forward because the gas is compressible and therefore the density is changing with the depth. There are iterative methods for calculating the hydrostatic pressure of gas, but these methods take some effort. With that said, someone once showed me a simple formula to estimate the hydrostatic pressure of gas, and it gets you surprisingly close to actual data. Here it is:
P-hyd-gas (psi) = 0.25 * (Depth [ft]/100) * (Surface Pressure [psi]/100)
The equation above works for 0.6 gravity gas and if you compare it to actual bottomhole data you will find that it's remarkably close.