Below is a study starting with 0 mana in each session casting non-stop Nullify Life Magic Self (no attempt to regain mana). 
In each case mana needed to cast ranged from 6 mana to 22 mana. I did not use a macro to do this, so further study 
could be beneficial. 

@ 330 Mana Conversion and 380 Life 
2 X 15 minute sessions (alternated between 300 skill to help adjust for lag) 
Spell: Nullify Life Magic Self 
Total cast attempts: 106 
Total successful: 106 
Average: 3.5 successful casts per minute 

@300 Mana Conversion and 380 Life 
2 X 15 minute sessions (alternated between 330 skill to help adjust for lag) 
Spell: Nullify Life Magic Self 
Total cast attempts: 85 
Total successful: 85 
Average: 2.8 successful casts per minute 

Comparison: @330 Mana Conversion Skill converted 25% more spells than @300 Mana Conversion Skill in a given time 
period with sub 22 Mana. 

Grandpa


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