TITLE: How to create a formula for calculating armor effect in a rpg game?
QUESTION [5 upvotes]: So I'm making a game and want to create an equation for calculating the effect of a stat called armor. The effect is in percent and determines how much damage reduction one has against attacks. The parameters are, 1, Player level and, 2, Armor points. Here's my initial approach:
I created a matrix $A$ that looks like this: 
$$\left[\array{
1 &  5 & 0.05\\
30 & 170 & 0.5
}\right]$$
I want the effect to be 5% when the player is level 1 and has 5 armor points. And when the player is level 30 and has 170 armor points, the effect should be 50%.
This all works fine, except for two problems:

The solution to that Matrix result in something unsuitable: When a player increases his armor points (without gaining in level) the effect decreases. If you have ever played a rpg, it should increase.
The changes to the effect are happening too fast. If $B$ is the effect when the player is level 25 and has 150 armor points and $C$ is the effect when the player is level 25 and has 151 armor points, $|B-C|$ is too big.

Any suggestions on how to get by problem 1 and 2?
To clarify:
I rowreduce matrix A. That gives me 
$$\left[\array{
1 &  0 & 0.3\\
0 & 1 & -0.05
}\right]$$
So the equation I use is
$$\text{effect} = 0.3\text{ level}-0.05\text{ armor}\;.$$
Here's what I would like to happen:
1. I would like the effect to increase as armor increases(and level standing still).
2. I would like the effect to decrease as level increases(and armor standing still).

REPLY [2 votes]: Try the following formula:
$$\mathrm{effect} = 1 - 5099/(\mathrm{level} \times \mathrm{armor} + 5098)$$
This assumes that both player level and armor points have a minimum of one. If not, add $1$ to each.
It has the correct result of a person at level 30 with 170 armor points giving an effect of $0.5$. However, it underestimates the result for a person at level 1 with 5 armor points. It has several advantages, though.  If both the level and armor points have the minimum value of 1, then it will produce a $0$. Also, no matter how high either value is, there will never be a result greater than $1$ (results greater than one would imply that attacks would actually boost a player's health).