If in fact the reading is accurate, then yes, 20% is too low. I would shoot for around 50-60% for a young russian.
Personally, I don't even measure the humidity in my baby russian enclosures. They are in a very dry room, but their 100 and 135 gallon tanks have several inches of damp coco coir, shallow water bowls and humid hides. They spend a lot of time in their hides or burrowed into the substrate, so ambient humidity in their enclosure or the room is of little concern to me.
Here is a care sheet with pics of their enclosures. Keep in mind they spend a lot of their waking hours outside in much larger enclosures, but they stay indoors at night and on cold overcast days. http://www.tortoiseforum.org/thread-80698.html