Max health on armor

you can use the tick triggers,

I know of a more complex way, but I'm not always great at explaining things and I won't be able to send images of it. But my way gives you more precise control over how much health each armor piece adds, allows multiple armor pieces (including from different sets) to stack together, and doesn't have the potion effect icon visible on your screen.

Step 1, create a new potion effect, in the attributes section, set it to increase max health by 1 (per level), in the main section, disable it showing up in HUD and inventory.

Step 2, create a new procedure to run on the global trigger of player tick update (click the green box at the top of the procedure). You will not need to link it to any of your armor tick updates, because it will already run every tick, and you will have it check for the armor later.

Step 3, create a local number variable (using the buttons on the left of the screen) and call it something like boost.

Step 4, add in an 'if' block with the condition 'not' 'is provided world client-side' this will prevent it from applying 'ghost' potion effects later.

Step 5, inside the not client-side if block, start the procedure by setting boost to -1.

Step 6, add in another 'if' block, again in the not client-side one, and set the new one's condition to be if get item from armor slot (number) of event/target entity = (armor piece). Using the red equals block and an empty red procedure block, as well as the get armor from slot block. Set the slot to be to type of armor (I'm pretty sure 3 is helmet, 2 is chestplate, 1 is leggings, and 0 is boots). (important, don't confuse get item from armor slot with get item from inventory slot).

Step 7, inside that new if block, set the local variable boost to get local variable boost + (the amount of max health you want to add), using the math operator procedure block (which has a lot of different options, '+' being one of them).

Step 8, repeat steps 6 and 7 for each piece of each armor set you have. This may take a little bit, but you should be able to do it fairly quickly by right-clicking on the if procedure block, then duplicating the entire thing, then changing the armor slot, the armor piece, and the boost amount. By the end of this, you should have a long column of if blocks, stacked on top of each other, but all inside the not provided world client-side one.

Step 9, add in one last if block, again inside the not provided world client-side one, and below the tower of if blocks for the armor pieces. Set this one's condition to be if boost > -1. You will need to get out the dark block '=' block, then click on the equals sign to change it to '>'.

Step 10, inside the last if block, add potion effect (the custom health boosting potion effect from step 1), with duration 3, level boost, and false for show particles and ambient (that way it won't be visible other than the health increase).

I have tried to make this as clear as possible, and as a result, it looks a lot more complicated than it actually is. It really isn't all that complicated.
 

Alternatively, if you want to use friyes's option, I would recommend using the vanilla health boost potion effect instead of absorption (health boost increases max health, absorption provides a shield that will reduce a flat amount from every hit you take), and maybe putting the procedure inside an if block with the condition of step 3 of my method, which will prevent a bug that used to happen (not positive if it still does or if Minecraft fixed it) that would make the potion effect never go away until you exit and re-enter the world.