Off the top of my head...
It would probably take me a week of reading just to refresh my memory with the physics and astronomy needed to do that calculation.! However, we do have a boundary condition we can use to help us with an approximation. The tidal effects on the rings induced by the object are comparable to those caused by Daphnis as seen in other images, so it is reasonable to assume the object is comparable to Daphnis in total mass. In fact, we haven't ruled out that the object isn't actually Daphnis, after the moon suffered some sort of breakup.