At level 29
I threw the fountain 4 onto a Priest/Rogue that's level 29 and fairly well-geared. I did some old-fashioned counting and then subtracted each tick of my mana bar from the next tick. I took five readings from my mana bar while sitting out-of-combat, standing out-of-combat, and being in-combat. I did all this while using the rune in a slot, and then again without the rune. I took five readings for each condition of regen to try to get a slightly more accurate reading than only one reading can give. Sometimes you'll get differences between ticks, and this could translate as an actual gain of fractions. Since your mana bar deals in whole numbers, it won't tell you that you gained 3.5 mana per tick, but it would translate a .5 gain by flip-flopping two differently sized ticks. This should become clearer as you read on.
You get back mana most quickly while sitting, and I found this to be my largest increase after slotting. Without the rune I was earning 53 mana per tick, but with the rune I received slightly over 68. One of the five ticks I measured was 69, which would suggest the overall average of a tick to be a fraction slightly over 68. The total gain is 15 mana per tick after the rune was slotted. That's a nice gain in mana regeneration from just one tier 4 fountain rune. I found standing while out-of-combat to be much closer to 26.5 before using the rune, but after slotting I was getting 34. I jotted down my guesstimate as 7.5 per tick. The smallest gain was while in-combat. Before using the rune, I was earning a little over 3 mana per tick. After slotting the rune, I was getting what appeared to be slightly under 4 per tick. More readings would have helped me, but I didn't want to spend all day taking readings. I scribbled down my gain as being a rough .5 per tick.
I definitely wasn't earning the labeled +22 from the rune, but getting an extra 15 mana per tick -- while sitting -- isn't too shabby.
At level 44
Care to take a guess at what the differences were on my level 44 Priest/Rogue? While my measurements are far from an exact science, I came up with just about the exact same figures. This means gear, level, stats and attributes aren't affecting the power of the fountain rune. It's probably a good thing too. If other stats boosted the rune's performance, it could be too easily min-maxed, and then what would we do with all those mana potions? As it is, there seems to be a lot of potential for some serious mana regen.
You are limited by being able only to slot it onto an accessory. But there are five accessory slots, and each one can have up to four rune slots. Even without being able to have duplicate runes,
you'd be able to slot four each of the fountain 10, 9, 8 and 7 runes you could slot up to a tier 10 in each accessory. Trying to imagine the possible mana regen from that sounds both cool and silly at the same time, though I've never tested above one tier 4.
Sometimes not all tiers seem to work in-game. The RoM Wiki has a transmutation guide showing runes and the tiers of each that Wiki writers have listed as tested, data, or disabled in-game. I'd take the guide with a pinch of salt, as it has been the same for a long time. It no doubt needs to be updated to reflect the current state of runes. I personally haven't seen anything above a tier 4 fountain in the auction house, and I've been on the lookout constantly. If you've seen any -- or own any -- let me know.
Who benefits most from fountain runes?
I immediately thought about using this on my Priest, but with the time and resources needed vs. the small return, it seems best-used on off-caster classes. Priests and Mages are already building large mana pools and stacking intelligence and wisdom stats. Adding some fountain runes probably isn't going to show an appreciable difference. On the other hand, a Knight/Priest that is stacking strength, stamina and defense stats will likely benefit more from slotting fountain runes. A Knight/Priest will have a smaller mana pool but still may use a lot of mana-based spells. In any case, fountain isn't going to help any class much in dungeons.
While running an instance, it may help you between trash mobs, but once you reach the boss, you're in combat through the entire encounter. You could try stacking a ton of fountain runes to eke out -- at best -- a tiny increase in mana regen, but at a huge cost. The time and money it would take to slot that many highly tiered runes just doesn't seem worth it. Then again, if you're like I am, you can be very obstinate and want to try it anyway.
Each Monday, Jeremy Stratton delivers Lost Pages of Taborea, a column filled with guides, news, and opinions for Runes of Magic. Whether it's a community roundup for new players or an in-depth look at the Rogue/Priest combo, you'll find it all here. Send your questions to firstname.lastname@example.org.