If there is such a point, I have not found it. In theory, yes, at some point I suppose things could get too wet. The big KEY here is your temperatures. As long as it stays 80 or above, all the time, day and night, humidity and moisture should not be a problem. If your tortoise was in a perpetual puddle, I would guess that could cause a problem, but short of that, you should be okay. And please remember, we are talking about starting hatchlings and babies, NOT maintaining adults.
Here's my question on tortoise supply website under leopard tortoise they wrote too much humidity can be harmful to leopards, do they mean adults? I have a 2 year old leopard that I adopted from a friend he is about 5 1/2 inches long and about 1 pound 3 oz. How do you guys house your 2 year olds or older? Do you worry about humidity or not after a certain age. Right now he is in for the winter, I have him on cypress mulch, with burmuda grass on top, and it's about 50% humidity at about 80-85 day and night with a 100 degree basking spot. Is that ok or would he benifit from a little cooler temps like say 75 or so degrees. I don't spray the enclosure at all, and soak him every other day or so? Just curious I can't find a care sheet for leopards that are not hatchlings.
If your method is working for you, I say keep doing it. Sounds fine to me. I think cooling into the 70's for an older one is fine with your lower humidity levels. Just watch for problems and make adjustments. Here where I live, they just move outside with a heated box once they are big enough. We have cold days (well, relatively cold, daytime highs in the low 50's and nights below freezing). They just stay in their warm boxes on these colder days and nights. Most of the year, temps are 70+, and my area has one of the highest sunny to cloudy day ratios in the whole country. (Learned that from my solar panel guy.)