How about some math?
Low Monster Defense (Brute & Artillery): Level+12
Med. Monster Defense (Skirmisher, Lurker, Controller): Level+14
High Monster Defense (Soldier) Level+16
Fighter Starting Attack Bonus:
4 (STR) + 2 or 3 (proficiency) + 1 (Weapon Talent) = 7 or 8
Rogue Starting Attack Bonus:
4 (DEX) + 3 (proficiency) + 0 or 1 (Weapon Talent) + 0 or 2 (combat advantage) = 7 or 8 or 9 or 10
Warlord Starting Attack Bonus:
4 (STR) + 2 or 3 (proficiency) = 6 or 7
Ranger Starting Attack Bonus:
4 (STR or DEX) + 2 or 3 (proficiency) + 0 or 1 (Prime Shot) = 6 or 7
7 is the most occurring and common number so we can start there and you can adjust by 1 (5%) as you see fit.
Base Attack Bonus vs Low Monster Defense = 7 vs 13
A 6 or higher is needed to hit, and a 5 or lower is needed to miss, so there is a 75% chance to hit and a 25% chance to miss.
Base Attack Bonus vs Med. Monster Defense = 7 vs 15
An 8 or higher is needed to hit, and a 7 or lower is needed to miss, so there is a 65% chance to hit and a 35% chance to miss.
Base Attack Bonus vs High Monster Defense = 7 vs 17
A 10 or higher is needed to hit, and a 9 or lower is needed to miss, so there is a 55% chance to hit and a 45% chance to miss.
Here is a probable attack vs defense progression chart where the main attack stat is always increased and magic item bonus to attack is increased at levels 3, 7, 13, 17, 23, and 27.
Code:
L A AC %
1 7 15 65%
2 8 16 65%
3 [B]9[/B] 17 65%
4 10 18 65%
5 10 19 60%
6 11 20 60%
7 [B]12[/B] 21 60%
[B]8[/B] 14 22 65%
9 14 23 60%
10 15 24 60%
11 15 25 55%
12 16 26 55%
13 [B]17[/B] 27 55%
[B]14[/B] 19 28 60%
15 19 29 55%
16 20 30 55%
17 [B]21[/B] 31 55%
18 22 32 55%
19 22 33 50%
20 23 34 50%
[B]21[/B] 24 35 50%
22 25 36 50%
23 [B]26[/B] 37 50%
24 27 38 50%
25 27 39 45%
26 28 40 45%
27 [B]29[/B] 41 45%
[B]28[/B] 31 42 50%
29 31 43 45%
30 32 44 45%
So if you fight a level 30 soldier at level 30 you will have a 35% chance to hit.