Resistances and Penetration

Question about resistances and penetration.

I am playing a Fire Burst Crit build (CI Elementalist), which procs the fire burst skill of the staff through ball lightning.

Due to the fact that i dont know how the resistance and penetration calculation works I am not sure if my setp is efficient.
The facts:

I use EE witch gives -50% fire res to mobs. (lightning dmg from ball lightning)
I curse the enemies with Ele Weakness witch is -39% fire res.
I also curse the enemies with flammability witch is -44% fire resist.

In total this gives the enemies -133% to fire resistance.
The first question is, what the basic resistances of monsters are and if I can reduce them below 0% similar to the players resis which can be below 0%?

Then I also use the Wise Oak flask witch gives 20% penetration to fire resistance.
I also use the fire penetration support gem for my fire burst skill witch gives 37% penetration.
And I will pick up Mastermind of Discord witch gives 25% penetration to fire resistance after using a lihtning skill (ball lightning).

This gives a total of 82% penetration to fire resistances.
My second question is, if it is efficient to have 82% pen if I already reduced the enemies resist by 133%?

How does all this get calculated in game?

My answer: Just download Path of Building, and let it make the calculations by itself. Is very easy to use, and there is a ton of guides of how to use it (use Google).

Resists can go negative... you're well into overkill range though.

I don't know either, but try here. This should give you idea of how much resistance mobs have. I think you only need to worry about ACT boss, map boss, side area boss.

http://pathofexile.gamepedia.com/Monster_resistances

As I can see, at least one source of penetration should be enough.

map bosses have 30% elemental resistances but there are many rare monsters that have very high resistances.

it's better viewed here http://poedb.tw/us/