With the Meditation skill you can regain mana two ways: passively and actively. Passive meditation is your base rate for regaining mana; you do not have to do anything special to trigger passive meditation, it is always happening. Active meditation is what happens when you actually use the meditation skill. While you are actively meditating, you will regain mana at a rate that is faster than your passive rate (up to the maximum rate of 2 mana per second). However, while you are actively meditating you cannot move, speak, cast a spell, equip/unequip items, or do just about anything else without ending the active meditation. Your chance to successfully actively meditate is based on the comparison of your Meditation skill against the difference between your max mana and your current mana. The more mana you need to regain, the harder it will be to actively meditate.

Meditation (both active and passive) only works if you are wearing no armor, leather armor, or armor with the "mage armor" property. Wearing just one piece of studded, ring mail, chain mail, bone, or plate mail armor will negate your entire meditation skill (and also negate your intelligence bonus, more on that below).

If you rely on mana as an important part of your play style, then Meditation is probably a stronger choice than Focus for you. 

Note: Meditation and Focus are both skills that affect the rate at which you regain mana. Meditation is more efficient at mana regeneration than Focus, but it also has restrictions for when it will and won't work. Focus always works for mana and also regenerates stamina, but Focus regenerates mana slower than Meditation. Which skill you choose, or if you want a combination of both, will depend on your play style and the number of magical items you want to wear. 
 

Calculating your mana regeneration rates /TOP
 

Points = 40*[INT(Focus/20)+(mana item total)]+intelligence+3*meditation
 

In the formula, "INT" stands for integer, not intelligence. Integer means round the result down to the nearest whole number. Basically what this is getting at is that for mana regen your Focus only improves in multiples of 20: 20, 40, 60, 80, etc. A Focus of 39 is exactly the same as 20. So the number you will end up with in the formula for Focus will be 0, 1, 2, 3, 4, 5, or 6.

For "mana item total" simply add together all the "pluses" you are wearing. For example, if you are wearing a hat with +1 mana regeneration and sleeves with +2 mana regeneration, your item total is 3.

If you are wearing ANY meditation blocking armor, even a single piece, set intelligence and meditation to 0 in the formula.

Once you have solved for "points," compare your answer to one of the following tables (depending on whether your Meditation skill is above or below 100):
If Meditation skill is less than 100

Points   Mana Rate (mana/sec)

  0                0.19
 40                0.29
 80                0.36
120                0.44
160                0.55
200                0.67
280                0.80
360                1.00
480                1.33
720                2.00

Points  Mana Rate (mana/sec)

  0                0.19
 40                0.29
 80                0.36
120                0.44
160                0.55
200                0.67
280                0.80
328                1.00
448                1.33
717                2.00

To achieve the maximum possible passive mana regeneration rate (2 mana per second), you would need 120 med, 120 int, and 120 Focus. Or 120 med, 120 int, and 6 +1 mana regen items. Or 120 Focus and 6 +2 mana regen items. For most practical character templates, 1.33 mana per second is about the best you can hope for.

The final thing to note is that on the above charts, there is no gradual progression. That is to say, you are either at .8 mana per second or 1.0 mana per second, but nothing in between. You can use this formula to figure out if wearing another mana regen item, or gaining 20 points in Focus, will really bump you into the next level or not.
 

Skill Training/TOP

Any time your current mana is lower than your maximum mana, you can gain skill in Meditation. The easiest way to raise skill is to sail forward on a boat while keeping your mana below maximum by casting spells, or by being drunk (while drunk your mana will occasionally drop). Casting spells will get you better overall results, since you can work multiple skills (magery, evaluate intelligence, chivalry, necromancy, etc.) while sailing. Meditation will gain in 8X8 runs, but since you are doing passive meditation you don't need to stop at each gain square; you will catch each gain as you sail past it. You can reasonably expect to get Meditation into the 80-90 point range in an afternoon or evening of play if you do nothing else, and can grandmaster the skill over a weekend. Individual results will vary, of course.