Not a silly question at all, a very thoughtful one.
Initially, I put him in the container use to soak him. I then poured water in slowy until it reached just above the bottom of his shell, about even with the bottom of the opening for his head and neck (think of him tucking in his head and letting it rest on his shell, that part). I then put a finger in the water to mentally mark where it rose to on my finger. Now I can just pour in the water before hand, using my finger as a sort of measuring stick. I think for Thor it's about 1/2" to 3/4", I'm not sure. Just remember those numbers might be different for your tort depending on his size. On a related note, one method I use to "wash" him is using a turkey baster to spread the water over his shell without making the water so deep that it would be uncomfortable for him. Hope all that helps
