Manta = A single unit of mana, 1 manta can power a luminus(Basic light spell)
Mantam = A single Manta being used for a minute straight, used to measure high-power spells(Simplified)
Mantahah = A single Manta being used over an hour, this used use to measure spells such a city-wide protective barriers and even mana-powered street lights(Simplified)

KEY
M - Manta
Mm - Mantam
Mh - Mantahah
m - Length of time in minutes
d - Length of time in hours

The Mantam usage of a spell is determined by Mm = M ^ (m/67.327); or for over an hour use Mh = M * ((d/5) + 4.12). This difference in measurement is due to needing to account for the ambient mana pressure increase(Where by using spells the amount of mana in the air increases, causing a spell to require less and less Manta. This can be a problem with long AND powerful spells, which can sometimes cause a "Mana event" a massive explosive release of mana. To decrease the chance of this you must carefully balance the strength of a barrier vs how much damage it takes(And thus mana it expells)).

Author note: This is the calculation used for an "average" mage with 5 years of experience and approx. 500 Mantahah regenerated per-day and a max capacity of 2500 Mantahah, if you specialize in either long term, or short term magic your experience may differ. Usage for a certain spell is not constant and can be changed by both tweaks to the spell and the one casting the spell.