Also remember that each spell has different coefficients of return for life steal or life on hit. some get horrible returns (cough blizzard) and some get great returns (DH traps) and most fall in between. This is especially true for AoE spells since they are designed to hit multiple mobs and I still haven't read anywhere what the "ideal" number is for everything to line up.
Life Steal has no coefficient aside from the reduction due to difficulty. If you do 100k damage with Spell A, you gain the same life as if you did 100k damage with Spell B. The only exceptions would be if a spell doesn't work with life steal, which hydra may not.
Also, the reported reflected damage is 10% of your damage done, which is further modified by armor and physical resistance. This means with 90% damage reduction you need 5% life steal to break even against a single target, after accounting for the reduction from Inferno. That sounds about right since I have 91% physical DR in Archon form, with 2.7+1.5 = 4.2% Life Steal plus almost 600 life per second regen and I gain life against 1 reflect damage mob. Expected life gain for 225k dps is 6057 life per second for single target, exected damage taken is 6075 damage per target.