Public Order

Disorder

Details here on what causes disorder effects and how they appear to be calculated

All settlements have a base public order level of 100%, all the effects that are applied are from this base level

Garrison

General concensus on RTW forums is that the garrison % is worked out as follows

Garrison % = garrison troop numbers * 700 / population size

This will work most of the time but as my own analysis of settlements indicated that this was not correct but is close enough, also you need to round down to the nearest 5%.

Unrest

Unrest is caused by a number of things as follows

Some settlements have a base level of unrest that you cannot remove, for instance most settlements in Spain have unrest such that if your faction is not the settlements initial owner then the unrest will always be there

capturing a settlement results in some initial unrest which typically reduces by 5% per turn.

When capturing a settlement it looks like the base unrest level is 30% when you occupy and 40% when you decimate the population

Enemy spies in your settlement cause 5% unrest per spy

Assassins can attack your settlement and cause damage and unrest

If your settlement is blockaded or laid to siege then this can cause up to 100% unrest