I' trying to solve for the surface temperature Ts=((n+1)So(1-A)4)1/4, and thus So=L/4𝞹a2 but I cannot seem to find the value for solar flux or its luminosity.
The solar flux at 1 AU (Earth's distance from the sun) is ~1361 W/m2. Flux falls off as the square of distance. Saturn, on average -- and thus Titan, on average -- is about 9.58 AU from the sun, so the falloff factor is 1 / 9.582 or 0.0109.
Titan should average about 1361 x 0.0109 = 14.8 W/m2 when not shadowed by the gas giant.
When Saturn is at its furthest from the sun, 10.12 AU, the flux would be closer to 13.3 W/m2.