A character may do one healing roll per 4 hours sleeping (or 2 hours meditation in case of elves).
HP healed = current HPs * (D10 + D10) / 100
The value is rounded to nearest. (If the fraction is less then 0.5 the value is rounded down. If the fraction is equal or greater than 0.5 the value is rounded up to the next larger number of hit points.)
(i) The amount of hit points a character may heal depends on his or her current condition. The more hit points a character has, the faster he or she may heal. (Of course only up to the maximum number of hit points).
(ii) Since the sum of two dice rolls is used, high rolls (20) and also low rolls (1) are very unlikely. Furthermore there is a higher probability that your roll results in a 10 than in a 1 or 20. This reflects that there is a higher chance to heal at a medium rate than very slow or very fast. Nevertheless the chance still exists.
(iii) Using 20% (20 / 100) of the current hit points as the maximum has the effect, that characters that are nearly dead (1 or 2 hit points) cannot heal naturally. They need some kind of treatment (e.g. heal spell, potion, etc.) before the natural healing process can start.
(iv) Even if natural healing is possible (3 hit points or above) a character may need several days to recover from severe damage (e.g. 3, 5, 8, 14, 25, 44 – based on the average healing rate)
* If you want that natural healing starts later (at higher numbers of hit points), you need to decrease the maximum percentage. You can decrease the maximum of the rolled value (e.g. use 2 D6 instead of 2 D10) or use a higher divider (200 instead of 100). I recommend that you keep at least two dice for the roll (see characteristic (ii)). When you use only one die, high and low rolls will be as likely as medium rolls.
* You can also change the rounding to achive the desired effect. Always rounding down will raise the boundary of hit points when natural healing is not possible. Always rounding up will eliminate characteristic (iii) and result in the fact that the characters gain at least 1 hit point per healing roll.