When I read the character Details it says, for example, “Fire Resistance: 1303” - Reduces Fire damage by 78.83% + 27.98%
I am not sure how this works? It is over 100% 
They’re not additive, but multiplicative.
Let’s imagine that a mob does an attack on the hero that would naturally do 10,000,000 damage.
The first damage reduction of 78.83% negates 7,883,000 of the incoming damage, leaving 2,117,000 damage to be dealt.
The second damage reduction of 27.98% negates 592,337 of the incoming damage, leaving 1,524,663 damage to be dealt.
This means the damage reduction was 100 - 15.24663 = 84.75337%
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OK, having checked a bit more, it appears the “+” component is given by the
Endless Walk set (ring + amulet) = while moving damage taken is decreased by 50%
and also
Captain Crimson’s Thrust set : Damage taken is reduced by % of cost reduction
The first component is all the resists on all the gear and talents I suppose.
Are you sure the calculation is performed in two steps? Sounds a weird system.
It’s always a bit difficult to know how the calculations are actually implemented in the code… for example, the coders could use an “equivalent elemental damage reduction” which would group the two numbers into one :
Equivalent elemental reduction = 1-(1-0.7883)x(1-0.2798) = 0.8475
The way Meteorblade presented it, however, is very useful to show how the various numbers combine… and, I think, much easier to understand than my little equation above 
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Don’t forgot that there are lots of other possible damage reduction modifiers that can take place and change through the game like your War Cry and Wrath of the Berserker, the possible effects of different legendary gems, the use of double Unity etc, etc. And then there are lots of possible buffs and debuffs from team mates when playing multiplayer. Everything that happens in the game is the result of millions of lines of computer code and constant calculation… both on the server and on the client computers.
The formula is;
Damage Reduced by Armor = ARMOR / ( 50LEVEL + ARMOR)
Damage Reduced by Resistance = RESIST / (5LEVEL + RESIST)
And as stated above damage reductions (as most things in this game) are multiplicative.
The same you will find with cooldown, resource cost reduction etc.
Example.
If you have 10% cooldown reduction from 1 source and add 10% cooldown reduction from another source you will have 19% cooldown reduction.
Why? Well just do it step by step, your cooldown is 100, minus 10% is 90, remove 10% from 90 (not from 100) and your left with 81…which means 81% of the original cooldown remains, which means that you removed 19%.
If you want easy calculation and building chars just check d3planner, lets you build chars but it can also download your current character.
If you download you character you can then copy it and change minor things (like re-roll one stat) and see how it impacts your character, before you do it ingame.
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I tried D3Planner once. It’s cool and all, but I don’t use it anymore. It removes a certain element of surprise and uncertainty from Diablo. Not knowing what’s going to happen or what you’re going to get back as a result of a choice is, I believe, a core component of Diablo 3…
Nooooooo
Not when it comes to things that you actually can have an influence on. 
I haven’t used d3planner to its fullest potential, only to things like to see what gave most damage on a weapon: a primary damage reroll or a % damage (re)roll and stuff like that.
Personally I love things like d3planner because I like planning, comparing and theorycrafting.
Without the planner I would either write a excel formula sheet to speed up theorycrafting (which I used before I used d3planner) or do it old fashioned way with pen and paper as I did in the time of d2.
Its like enjoying a game while not playing it at that time and it was much more fun in the D2 time (much more diversity), but even in D3 it can be a bit of fun, especially when new stuff arrives.
I may still have the old spreadsheets with weapon attack speed calculations, something about frames per swing or something, something about breakpoints, I vaguely remember I had a zealot paladin or what’s it called… I remember there was a mace (Schaefer’s Hammer maybe) but there was an issue with range on the weapon, and when being ganked it helped if range on the weapon was +1.