Default accuracy used in character sheet chance to evade calculation?

What is the default monster accuracy used in the chance to evade calculation for the character sheet? I am trying to create a damage reduction/avoidance spreadsheet, but before I can make the monster's accuracy customisable, I need to have a baseline. Since the chance is reported as a rounded percentage, it's difficult to work out what specific value is used.

I only have a level 30 character to test on, but here are my data points:

36% chance to evade, 590 armor
.36=1-x/(x+(590/4)^0.8)
96.59 accuracy rating

34%, 548
.34=1-x/(x+(548/4)^0.8)
99.41 accuracy rating

24%, 284
.24=1-x/(x+(284/4)^0.8)
95.85 accuracy rating

18%, 192
.18=1-x/(x+(192/4)^0.8)
100.82 accuracy rating

Edit: I assume having data from only a level 30 is useless, because the only variable under control of the system is monster level. Adding data from other random characters I have:
Lvl 40: 20%, 712
.20=1-x/(x+(712/4)^0.8)
252.57
Lvl 40: 14%, 421
.14=1-x/(x+(421/4)^0.8)
254.77

Lvl 1: 31%, 55
.31=1-x/(x+(55/4)^0.8)
18.12

Lvl 33: 46%, 1245
.46=1-x/(x+(1245/4)^0.8)
115.91

Lvl 8: 41%, 160
.41=1-x/(x+(160/4)^0.8)
27.52


Anyone more mathy than I who can work out the formula for monster accuracy used in character sheet chance to evade based on the above? Or do you need more data points with different levels? Or else, am I just a fool who can't find a source that already knows what the base accuracy rate used is?

Thanks in advance,

Path

Posting those data points into a random linear regression calculator, I get a formula very close to:

monster accuracy=8+(5*level)

Can anyone either post me their expected monster accuracy using the above formula, or else anyone who can post me their character sheet evasion rating, chance to evade and level would be incredibly helpful! thanks!

edit: clearly not right given lower numbers at lower levels! I assume it isn't linear :(