Mana Font (Chalice Unique Passive) giving too much mana

The tooltip for mana font says that it returns 1.5% missing mana every 5 seconds. It's currently giving far more than that. Not sure if it's supposed to give 1.5% mana per 5 or 1.5% mana per 1 but it would be nice to have some clarity here.

I'll copy paste my more detailed post below for better visibility:

Okay I went ahead and took a screenshot to prove my point here, and before you ask - No I was not using one of the mana potions in my inventory. Those were bought so I could farm up the chalice faster. Turns out I didn't need em. And before you ask, I took my AD runes to make sure I didn't have mana regeneration seals. Because all my ap pages have 'em xD And I took 21/9/0 masteries. So no MP5 from the utlity tree.

http://imgur.com/wbpQ2iv

Sorry it's low quality, I snapped it on my phone. League doesn't have a windowed mode for easy pictures and I didn't wanna take the time to figure out how to screen shot for this. In the picture:

I am level 7 with 576 mana. At this point my "base" MP5 is 10 (roughly). There are 2 items in my inventory - Chalice and Dorans Ring

Dorans Ring gives 3 flat MP5. Chalice is supposed to add 50% of my base (Brings 10 to 15) and gives Mana Font "Restore 1.5% of missing mana every 5 seconds".

So lets do the math.

Without mana font have roughly 18 MP5 (10 base + dorans ring + 50% from chalice) I am currently have 126 mana remaining out of a total of 576. The difference is 450.

Based on this information Mana Font should give me 450 0.015 = *6.75 mana every 5 seconds at this value

So lets add 6.75 MP5 and 18 MP5. What do we get?

24.75 MP5.

What does my screen show?

42 MP5

WTF?

The only way this could be possible is if Mana Font is giving a ridiculous amount of mana more than it states.

Not sure how or why, but it's giving close to 3 times as much as it should. It should be fixed.