For me, this comes down to limiting the deflection of the facade. If the building swayed 25 mm, you check that against a facade deflection of whatever the limit is, like L/400. I'm not sure how an FEM model would come into play unless it was a curtain wall or other engineered system.
Usually, it's the other way around. The FEM model "produces" the sway, and the facade deflection is checked if it's exceeding this or not. 25 mm is a very low amount of sway, especially when considering things like deflection amplification with seismic loading. They might be asking you to limit the whole building deflection to that much, which is a difficult task.