The Mononobe-Okabe equations do not apply to restrained walls where activation movement is restricted.
For seismic lateral on restrained walls Retain Pro uses P = (soil density) x (k<sub h>) x (retained height)^2. Application is approx. 0.60 x (retained height). This is a slightly trapezoidal loading but with only slight error a uniform loading could be assumed. K<sub h> is the acceleration factor appropriate to the site.
See Kramer, Geotechnical Earthquake Engineering, Section 11.6.2.
Hugh Brooks, SE
Retain Pro Software