Sadly there are not much datasheets for 18650 available.
I am looking for a samsung 20R datasheet quite for a while and asked here and there but nobody seems to have one. So nobody knows for sure how deep these can be discharged.
You can have a look at the discharge graph for the 20R from HJK. Especially under high load there is not much capacity left below 3.3-3.4V. If the 25R behave similiar that would be a good value to dim down.