I'll add to that that in 90% of the cases both pieces are completely similar in bonuses given and count for half of your total defense, not counting trinkets and UV's (your shield's resists and defenses only count for when it's up and how quickly it'll break). The remaining 10% are generally other bonuses, like the Volcanic bomber helmet giving CTR: medium while the Volcanic bomber suit gives CTR: low.

Click on the Knight (horse head) icon and look at your Character Window. Down at the the bottom there is a Helmet icon which totals up your total defense of Helmet + Armor + trinkets (if any) and lets you know what your defense is (more important when mixing and matching armor sets).

Trying is referring to the way the values are shown at the bottom of the Character Panel. A full bar of defense on an item will only show half a bar of defense on the totaled amount. So if an armor and helm both have full bars then the totaled amount will also be full something like:
(1 + 1) / 2 = 1 where "1" is "full"

This is in contrast to status resistances which are simply additive. 3 freeze bars from the helm and 3 freeze bars from the armor will result in 6 freeze bars total.

Trinkets work the same way. A defensive trinket's bar is cut in half, a resistance trinket is fully added to the bar's length.

no it doesnt, and just imagine that the defenses get added up, dont need to complicate using formulas lol, some helmets give different bonuses than the armors on their set so you still have to do some research.