Ok, I'm a little rusty with the actual mechanics.
With Shelter and Guard, a percentage of the damage incoming is diverted to the nanopool. The damage converted into nanopool, is greater than the damage you would have been hit with. So, say, Shelter "reflects" 25% of the damage you would have been hit with and say that 25% = 1k. Your nanopool would be hit for more than 1k, how much depends on breed.
The biggest advantage to Shelter, is you will never be 30% capped for your HP, because it works with the damage -after- the cap has been calculated. It's why an NT can stand in an OS and not die, but be left with a little HP remaining.
That's why pre Izgimmer's Wealth, NTs still favoured low HP setups and found it necessary to dodge items that add HP, to keep NBG useful, because the more HP, the bigger the 30% hit and thus the more nanopool got chewed away at a greater ratio then the actual damage dealt.
The actual numbers for how NBG (the decent DtN "absorb"):
If Atrox, 80% of the incoming damage is converted into your nanopool at a rate of 200%. So say 2k damage that would have hit you, is directed to nanopool, you'd lose 4k nano.
If Solitus/Opifex, 85% of the incoming damage is converted into your nanopool at a rate of 175%.
Nanomage is 90%/150%. So as you can see, pre IW, especially for Atroxes, NBG would zap your nano pretty fast if you were heavy on the HP side of things in the face of capping specials. I believe someone worked out the optimum HP at something stupid like 5-6k HP, anything past that reduced the effectiveness of NBG, or basically it would never last for the full 25s.
For Shelter, the "reflect" nano, it is 25% of damage dumped into nanopool across all the breeds. The conversion rates of damage:nanopool are:
1:1.5 for Atrox
1:1.15 for Solitus/Opifex
1:1 for Nanomage.
So for every one point of damage dealt, 1.5 points hit's the Trox pool, 1.15 points hits the Soli/Opi pool and 1 point hits the Nanomage pool.
I hope this makes sense.