Once you shut in the well in and obtain shut in drill pipe pressure, you can estimate formation pressure by applying the hydrostatic pressure concept.
This following equation demonstrates you how to figure out formation pressure from the kick analysis.
Formation Pressure = SIDDP + (0.052 x Hole TVD x Current Mud Weight)
Formation Pressure from Kick Analysis
Unit of each parameter is listed below:
Formation Pressure in psi
SIDDP (shut in drill pipe pressure) in psi
Hole TVD (true vertical depth) in ft
Current Mud Weight in ppg
Example: Well depth = 8,500’MD/8,000 TVD.
SIDPP = 300 psi
Current mud weight = 10.0 ppg
What is the formation pressure in psi?
Formation Pressure = 300+ (0.052 x 8000 x 10.0)
Formation pressure = 4,460 psi
Ref book: 
Formulas and Calculations for Drilling, Production and Workover, Second Edition
