How do you create a balanced "restore mana" ability?

[Verdun]

I made previous posts about my mishaps regarding trying to make an ability that "refuels" units by using its own mana. Spirit Touch seems obvious but sadly it's very buggy - for instance no matter what unit classes you set it to, it will never autocast on mechanical units (you have to apply it to mechanicals manually), that seems to be hardcoded. Same for any variant that involves healing abilities plus triggers.

An alternate route I figured out was to repurpose that mana scroll item ability, doesn't have autocast but at least you can make it restore mana to a group of surrounding units without discriminating against mechanicals. Not ideal but somewhat better.

My question is, how do I now make this balanced for gameplay purposes. Say I give it a range of 300, how much mana should it restore, at what mana cost?
If say the "Gas Station" Unit has 100 mana, does it make sense to make the ability cost 100 mana and restore a max of 100 mana to every surrounding unit?
But that has the drawback that if you got only one unit around that only needs 10 mana restored, you just wasted 90 mana. Likewise, if you got 20 units around who get a complete refuel, you just restored 2000 mana for the price of 100.

Any ideas what a good ratio would be here?

 

[A]mun]

Probably giving the gas station much more mana, then decrease that per individual unit - pseudo realistic.
Like moon wells function, just for mana only.