How add npcs/items/spells into a game using math?

[BeNjO]

I have been thinking about doing the in-game stuff my self for a few months now, but one thing i can not seem to be satisfied with, is how much to increase certain items, and make them fair for all classes? Is there a easier way of adding them without having to vigorously test every item against other classes, and the NPC's for said items level? I'm just at a loss on how to make the in-game database without spending most of my time testing every small adjustment over and over.

[Agoraphobic]

What I normally do when I first start is to create a min/max chart for the following;
_Hp, Damage done by attacks, Damage done by spells, Armor, Parry, etc._

This allows you to get a good idea what is the bare minimum, the average, and the absolute maximum that can happen for each class. This is a good starting point, however only through testing will you make sure everything is close to 100% correct.

Example of a HP min/max chart. (It's not wanting to align right, added the attachment).

>! HP Averages HP = (Vitality x100) + (Strength x 25) 
>! Level Stat Min         Stat Avg        Stat Max          HP Min      HP Avg    HP Max
1 Stat: 3 Stat: 5 Stat: 13 375 625 1375
2 Stat: 4 Stat: 8 Stat: 24 500 1000 2500
3 Stat: 5 Stat: 11 Stat: 35 625 1375 3675
4 Stat: 6 Stat: 14          Stat: 46 750 1750 4800
5 Stat: 7 Stat: 17 Stat: 57 875 2125 5925
6 Stat: 8 Stat: 20 Stat: 68 1000 2500 7050
7 Stat: 9 Stat: 23 Stat: 79 1125 2875 8175
8 Stat: 10 Stat: 26 Stat: 90 1250 3250 9300
9 Stat: 11 Stat: 29 Stat: 101         1375 3625 10425
10 Stat: 12 Stat: 32 Stat: 112         1500 4000 11550
11 Stat: 13 Stat: 35 Stat: 123         1625 4375 12675
12 Stat: 14 Stat: 38 Stat: 134         1750 4750 13800
13 Stat: 15 Stat: 41 Stat: 145         1875 5125 14925
14 Stat: 16 Stat: 44 Stat: 156         2000 5500 16050
15 Stat: 17 Stat: 47 Stat: 167         2125 5875 17175
16 Stat: 18 Stat: 50 Stat: 178         2250 6250 18300
17 Stat: 19 Stat: 53 Stat: 189         2375 6625 19425
18 Stat: 20 Stat: 56 Stat: 200         2500 7000 20550
19 Stat: 21 Stat: 59 Stat: 211         2625 7375 21675
20 Stat: 22 Stat: 62 Stat: 222         2750 7750 22800

[BeNjO]

will give this a shot, and maybe make a program using your table as reference for easy input, how would you go about changing the table per class? instead of x str use x int/will/end depending on class?