Let the following variables:
OMC = Original mana cost
WMC = Wanted mana cost
UA = Useanimation
UT = Usetime (interval between shots`
SPU = The amount of shots per use
Changing the mana cost requires following shot per use ratio:
SPU = OMC/WMC
The amount of shots per use (SPU) is UA/UT round up. <=> UA = SPU * UT.
E. g. you want to reduce the demon scythe’s manacost from 14 to 9 the SPU ratio is 14/9. So UA = 14/9 * UT.
That means you can have eg. a UT of 18 and UA of 28.
Note that you have to have the OMC of mana to use that weapon at all.