That's a fine way to look at it, assuming you have valid U and SHGC values for your UGUs modified with added frit. You can derive these yourself using the Optics tool, but many glazing manufacturers will also have some listings for their products with and without frit for comparison.

FWIW, some manufacturers have published performance data for glazing products with the addition of frit at various densities. One helpful tool to study the influence of frit on SHGC is the Vitro Glass Constructor. You can construct single, double, and triple pane assemblies with the IGDB and prescribe frit patterns to certain company's products. For instance, the Vitro products offer many different frit densities whereas Interpane has none. Old Castle also offers reporting data for glazing products with three different densities and patterns for frit. From what I have seen, the overall contribution to SHGC depends on the substrate glazing but can range from 0.02-0.05 decrease in SHGC with 10% increase in white frit density. Intuitively, white frit will perform better for SHGC but I have not come across any grey frit performance data yet.