Hi. I am proficient user in Staad Pro and have used it for both theoretical models and design applications, particularly with reinforced concrete as a material. What I understand from your description is that you are only interested in the analysis results ( bending moments, shear forces, stresses, reactions ..) on the slab on grade which I assume is mat foundation supporting set of columns with determined loads. There are many techniques to accurately model this type of foundation in regards to spring stiffness module, meshing, column boundaries to avoid singular stress values ...etc.
I can assure you that I have the experience in experimenting with these modelling technique and can give you results with confidence that are not only fit for practical purposes, but accurate enough to the theoretical values. I understand that you need this assignment to be urgently accomplished and I am offering 24-hour completion time with brief report. If all that sounds good then just let me know.
Best regards,
Hattem