This is a significantly more complex problem than you seem to realize.
First, your unity check equation above is incomplete. The moment will cause a deflection of the member. The axial load acting on this deflected shape will create an additional moment. This effect, called (logically enough) moment amplification, must be taken into account in the unity check.
Second, the allowable axial stress (Fa) will be a function of the restraints at the ends of the arch and within the structure. Depending on the configuration of the structure, buckling might occur locally (i.e., between points of bracing withing the structure) or globally (buckling of the overall arch).
I think that this is a difficult enough problem that you probably need to get the assistance of a qualified structural engineer. You might try posting this question on the civil engineering group (sci.engr.civil). There are a number of building and bridge oriented consulting engineers there who may be able to help.