His shield scales with health better than any other shield in the game 1:1. No it's not health until he needs it to be.
As far as tahm kench is concerned he gets AD from his ult. Yes it's magic damage, but he doesn't have any scaling with AD, so the only the it effects is his raw ATTACK DAMAGE
So the basic formula for damage resistance would be... D x (1-R) = H
Where, D is the unmitigated damage, R is the resistance, and H is the mitigated damage. Meaning R = H/D - 1.
To prove this calculation i'll use Mao as an example (because of his simple 20% damage reduction). R = 20%, D = 75%.
.75 x (1 - 0.2) = .6 [D x (R - 1) = H]. H = 60%.
To confirm that calculation let's confirm what the damage resistance was.
H/D - 1 = R,
.60/.75 - 1 = 0.2 or 20%. So for tahm, say he doesn't activate his E in a trade, we can use this same formula to calculate how much damage he would take.
Say he has rank 1 E (20% grey health healing), and takes 75% damage, he would then heal for .75 x .2 = .15, this means he would regen back up to .25 + .15 = 40%. So the unmitigated damage taken was 75%, but the final damage taken would have been 60%. If you don't believe me, i'll calculate it as if he had raw damage resistance instead of this regen mechanic.
.75 x (1 - .2) = .60 <<<< mitigated final health
(.6/.75) - 1 = .2 <<<< effective damage resistance
The numbers are identical, to if he would have had the raw damage resistance anyway. So if you poke tahm, or if you fail to force him to E, then he effectively has 20 / 26 / 32 / 38 / 44% damage resistance.
tl;dr... because i know most people who play league aren't physics majors... D x (1-R) = H, is used to calculate both raw damage mitigation (e.g. Mao) and tahm's E healing. Ergo, they can be considered the same.