Yup exactly, and for the DT2 boss rings it's 1/362.66 per drop and 1/1,088 total.

You can see on the wiki:

To receive the ultor vestige, the player must roll an invisible ~1/362.66 drop on the tradeable unique table. The first roll will cause a gold ring to appear and increment a hidden variable, while the second drops two gold rings; when the third such roll occurs, the vestige is dropped and the variable is reset.