Parabola suggested it may have to do with binary values. 0.4 seems to start working at 128.4, and 0.8 starts working at 256.8. Maybe the way to test this further would be trying with x.2 below 128:

So from the Unlimited Concentration Casting discussion, Teccam reckons it also works at .8 over Fortify Magicka 155.

And about Vampires, I'm fairly sure the bonus wouldn't work during the day, since you'd have negative attributes.

EDIT: The other problem with Necromage is that it'll also enhance the Fortify Restoration potions. So you'd need to luck into a completely new combination that only works with Necrovamps.

@BloodBane, or anybody else who happens to be able to test this (since Ning won't let me reply to specific posts, I'm just replying to the thread in general).

Try Elixir to Draught with +50. That results in 30.46875 if my maths is correct. That's the closest I've been able to get to .4 so far. EDIT: Just realised the item will have to be a +40 to begin with, since Necromage will enhance it by 25%. Jesus BloodBane, this isn't an easy ask :P

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Just to clarify with the table at the bottom. Does the +40,+20,+70 apply to the potion/draught/philter? or to the size of the stamina apparel?

The +40, +20, +70, etc. all apply to the magnitude of the Fortify Stamina enchantment on your item/blessing/etc. Potions, Draughts, and Philters of the Healer all have preset values that cannot be modified by the player. You just buy those at an alchemist shop in any town and they will be the correct magnitude every time.

I tried the Philter, Draught with a +21 custom enchanted ring but it didn't take. Does the item have to be exact value or is it likely something else may be off?

The enchantment has to be the exact strengths listed, otherwise it doesn't work. Really you'd be better off just buying a ring of minor stamina since that is exactly 20. 

Ok I'll do that. I saved just before trying it in case it didn't work out so no potions were wasted. thanks

Potion, Draught with +20 and +20 together  – can be achieved at lv10 (as draughts become available at this lv).

Lemme see if I understand this correctly. In the example above I would use a philter/draught of the healer +20 and then equip an item that gives 20 stamina to gain infinite stamina?

No, you drink a Potion and then Draught of the healer and then equip two items of +20 stamina to gain infinite stamina