I have a 75% reduction on my gear so I can use Expert fire 20 times, or like what I actually do and use half the bar conjuring a storm atronarch, I have no perks in that tree so it is very costly, then I have a 25% reduction in Alteration so I can cast Ebony flesh with half magica bar, and just to help cast other spells in that school. Also it increases the number of uses I can get out of a staff of paralysis from 10 to 15, again no perks in Alteration and the staff is used only in very tough situations. No reduction on restoration because I don't use wards just healing spells, even with the perk in that tree a steady fast ward is not magica sapping.
Then illusion, I am level 52 with two spare perks and none in illusion, I want to put some into that school so I end up using it more. I don't know whether I need perks or not, I have two slots of enchant left on armour so I could get a 50% reduction, with no reduction perks. Would this be good enough, or should I also get a perk for reduction. I would not like to use a staff. I would then need about 10 perks in that tree or if no reduction perks is needed then 7.
Also yes I did take the 100 smithing and Enchanting route. Not to create an OP playstyle, my armour rating is only 300 odd.