What is the amount of damage resisted for every one armor point increased? Is it a percentage or a set number?

http://www.swcombine.com/rules/?Ground_Combat#Base_Damage_and_Armor
The percentage of damage your armor absorbs, or prevents, depends on the firepower of the weapon you are attacked with. The maximum firepower is 24 so if you plug that into the equation of (Firepower / Firepower + Armor) you can get the minimum percentage of damage absorption to expect from a specific armor value. Note that as the firepower of a weapon decreases the amount of damage your armor absorbs increases.

Ah, I missed the equation part. Much appreciated!

Wow, that is absolutely beautiful! Thank you very much Tomas!

Just thought I would add some weapon specific examples with some common armour ratings. The differences in damage output between the Elg-3a and the fragstorm shotgun (with the same firepower rating) are due to the different range of damage ranges that each weapon care do. That is the reason for the damage ranges after each armour rating instead of a simple percentage of damage reduced. These differences can be minimized by a combatant with a higher projectile weapons score.

A-295 with a firepower of 12 hit a player with a Corellian powersuit and a helmet (armor rating of 15) we would expect 44.4% of the damage from the attack to get through (12/27=.444 or 44.4%).

If the attack would have done 7 points of damage (the A-295's max damage for each of it's 6 shots) to an unarmored target then we would expect it to do 3.1 points of damage to the target with 15 points of armor.

If all 6 of the A-295's shots would have done 7 points of damage to an unarmored target then we would expect it to do a maximum of 19 points of damage to a target with 15 points of armor.

An A-295 with a firepower of 12 can expect to get this percent of it's maximum damage of 42 against the following armor ratings:

-5= 70.5% or about 12-30 HP
-7= 63.1% or about 11-27 HP
-10= 54.5% or about 10-23 HP
-12= 50% or about 9-21 HP
-15= 44.4% or about 8-19 HP
-18= 40% or about 7-17 HP
-20= 37.5% or about 7-16 HP
-22= 35.3% or about 6-15 HP
-27= 31% or about 6-13 HP

A Bowcaster with a firepower of 13 and max damage of 45 will do the following against these armor ratings.
-5 = 72.2% or about 21-33 HP
-7= 65% or about 19-29HP
-10= 56.5% or about 17-26 HP
-12= 52% or about 16-24HP
-15= 46.4% or about 14-21 HP
-18= 41.9% or about 13-19 HP
-20= 39.4% or about 12-18 HP
-22= 37.1% or about 11-17 HP
-27= 32.5% or about 10-15 HP

ELG 3-A fire power 18 and max damage of 60 vs Armor rating
-5 = 78.3% or about 31-47 HP
-7= 72% or about 29-44 HP
-10= 64.3% or about 26-39 HP
-12= 60% or about 24-36 HP
-15= 54.5% or about 22-33 HP
-18= 50% or about 20-30 HP
-20= 47.4% or about 19-29 HP
-22= 45% or about 18-27 HP
-27= 40% or about 16-24 HP

C-M Frag Storm Heavy Shotgun - firepower 18 and a max damage of 60 vs AR
-5 = 78.3% or about 23-47 HP
-7= 72% or about 21-44 HP
-10= 64.3% or about 19-39 HP
-12= 60% or about 18-36 HP
-15= 54.5% or about 16-33 HP
-18= 50% or about 15-30 HP
-20= 47.4% or about 14-29 HP
-22= 45% or about 14-27 HP
-27= 40% or about 12-24 HP

A-280 firepower 20 and max damage of 65 vs AR
-5 = 80% or about 36-52 HP
-7= 74.1% or about 33-49HP
-10= 66.7% or about 30-44 HP
-12= 62.5% or about 28-41 HP
-15= 57.1% or about 26-38 HP
-18= 52.6% or about 24-35 HP
-20= 50% or about 23-33 HP
-22= 47.6% or about 22-31 HP
-27= 42.6% or about 19-28 HP

Nightstinger fire power 24 and max damage of 85 vs AR
-5 = 82.7% or about 41-71 HP
-7= 77.4% or about 38-66 HP
-10= 70.6% or about 35-60 HP
-12= 66.7% or about 33-57 HP
-15= 61.5% or about 31-53 HP
-18= 57.1% or about 29-49 HP
-20= 54.5% or about 27-47 HP
-22= 52.2% or about 26-45 HP
-27= 47.1% or about 24-40 HP

These numbers may not be exactly right due to the specific way that combine does its rounding but they are close to my real world experience.