I have a level 40 cleric, with a meditation skill of 205.
every tick I receive 17 mana, then drops 1, and then jumps 17 mana on the next tick, giving me an effective regen rate of 16 mana per tick.
after reading the meditate skill definition:
"For every 12 points your Meditate skill has, you will regenerate +1 mana (per tick) when sitting."
source
http://wiki.project1999.com/Meditate
204/12=17 +1 should equal 18 mana per tick.