Refer to Adrien Pages work from the 1980's his stuff is best.

I'm going to copy the answer that I got long time ago from CSI.

"For shear wall (both piers and spandrels), the flexural and axial behavior is modified by either f11 or f22 depending on the orientation of the local axis and the shear behavior is controlled by f12. In column and code terms f11 or f22 would correspond to modifications of EI or EA and f12 would correspond to modifications to GAshear. The code recommendations in Section 10.10 of ACI 318 code are related to slenderness effects where flexural deformations govern so they have recommended modifying EI (corresponding to f11 or f22 for shear walls). There is no recommendation about reducing the  GAshear. You should, however, note that some of our users use modifiers for f12 also, where they expect deterioration of shear stiffness and want to be realistic in their modeling. For slabs where bending is always in the out-of-plane direction, modifiers m11, m22 and m12  are required to model cracking behavior."

I think the process is iterative as you should start by assuming a section modifier then use the results to perform a section analysis and get a new value of the depth of the uncracked part of the section then get the new modifier and input it, this process should be repeated until reaching a nearly constant value of the modifier.

hi i have recently answered a similar question , i give u the same answer hope it will be what you have been looking for

I believe you should first analyze your structure by assuming the said parameters as 1 , after the analysis check the stress contours for both , if the stress surpassed the the concrete strength you should then apply the modifiers which are given in codes ,for instance ACI suggests 0.35 and 0.7 for bending and compression

this is what we do in a normal concrete shear wall i suggest check if there was a finite element model about reinforced masonry wall,

or model 1 for yourself and and check stiffness degradation then you use your own modifiers since it has research backgrounds

sorry for the long comment hope it was helpfull

best regards