Sorry for the late reply, been swamped with school, but thought I'd post this before it becomes super outdated (I actually had most of these values figured out much last month)
The below are the final values I tested with that I'm fairly certain work well. I made some changes to traits to modify the acumen bonuses (tweaking for balance) and others that I mentioned earlier but that's not complete in any form and currently will crash the game so I won't be posting that yet, unless your team would want it? I can write what I intended to do, I don't really have much time for modding due to classes. :/
I'm not sure 100% that the highest level city can be reached yet but I know I was still getting >1% positive pop growth values at 53k pop with not that great governors. To go from a large town to a city, you will need a good governor (there's tons of characters on the map you can bribe or start with that have good acumen, I personally didn't have issues with good governors), but since city's pop growth buildings give a huge bonus, you don't need one after that.
Code:
<?xml version="1.0"?>
<root>
<factor_modifiers>
<factor name="SPF_FARMING_LEVEL">
<pip_modifier value="1.5"/>
</factor>
<factor name="SPF_FARMS_BUILT">
<pip_modifier value="1.75"/>
</factor>
<factor name="SPF_HEALTH">
<pip_modifier value="1.25"/>
</factor>
<factor name="SPF_BUILDINGS">
<pip_modifier value="1.0"/>
</factor>
<factor name="SPF_TAX_RATE_BONUS">
<pip_modifier value="2.0"/>
</factor>
<factor name="SPF_BUILDINGS_FUN">
<pip_modifier value="0.25"/>
</factor>
<factor name="SPF_GOVERNORS_INFLUENCE">
<pip_modifier value="0.25"/>
</factor>
<factor name="SPF_TRADE">
<pip_modifier value="1.25"/>
</factor>
<factor name="SPF_SQUALOUR">
<pip_modifier value="0.825"/>
</factor>
<factor name="SPF_PLAGUE">
<pip_modifier value="2.0"/>
</factor>
<factor name="SPF_TAX_RATE_PENALTY">
<pip_modifier value="2.0"/>
</factor>
<factor name="SOF_GARRISON">
<pip_modifier value="2.0"/>
</factor>
<factor name="SOF_BUILDINGS_LAW">
<pip_modifier value="1.0"/>
</factor>
<factor name="SOF_BUILDINGS_FUN">
<pip_modifier value="0.5"/>
</factor>
<factor name="SOF_GOVERNORS_INFLUENCE">
<pip_modifier value="0.025"/>
</factor>
<factor name="SOF_TAX_BONUS">
<pip_modifier value="1.0"/>
</factor>
<factor name="SOF_TRIUMPH">
<pip_modifier value="1.0"/>
</factor>
<factor name="SOF_BOOM">
<pip_modifier value="0.25"/>
</factor>
<factor name="SOF_ENTERTAINED">
<pip_modifier value="0.75"/>
</factor>
<factor name="SOF_HEALTH">
<pip_modifier value="0.5"/>
</factor>
<factor name="SOF_FEAR">
<pip_modifier value="0.10"/>
</factor>
<factor name="SOF_GLORY">
<pip_modifier value="0.5"/>
</factor>
<factor name="SOF_SQUALOUR">
<pip_modifier value="0.85"/>
</factor>
<factor name="SOF_DISTANCE_TO_CAPITAL">
<pip_modifier value="0.5"/>
<pip_min value="0"/>
<pip_max value="15"/>
</factor>
<factor name="SOF_NO_GOVERNANCE">
<pip_modifier value="0.5"/>
</factor>
<factor name="SOF_TAX_PENALTY">
<pip_modifier value="1.75"/>
</factor>
<factor name="SOF_TURMOIL">
<pip_modifier value="1.5"/>
</factor>
<factor name="SOF_BESIEGED">
<pip_modifier value="0.5"/>
</factor>
<factor name="SOF_BLOCKADED">
<pip_modifier value="0.5"/>
</factor>
<factor name="SOF_RELIGIOUS_UNREST">
<pip_modifier value="1.5"/>
</factor>
<factor name="SOF_EXCOMMUNICATION">
<pip_modifier value="2.0"/>
</factor>
<factor name="SIF_FARMS">
<pip_modifier value="0.75"/>
</factor>
<factor name="SIF_TAXES">
<pip_modifier value="0.35"/>
</factor>
<factor name="SIF_MINING">
<pip_modifier value="0.75"/>
</factor>
<factor name="SIF_TRADE">
<pip_modifier value="0.5"/>
</factor>
<factor name="SIF_BUILDINGS">
<pip_modifier value="1.0"/>
</factor>
<factor name="SIF_ADMIN">
<pip_modifier value="1.0"/>
</factor>
<factor name="SIF_WAGES">
<pip_modifier value="1.0"/>
</factor>
<factor name="SIF_UPKEEP">
<pip_modifier value="1.4"/>
</factor>
<factor name="SIF_CORRUPTION">
<pip_modifier value="0.25"/>
</factor>
<factor name="SIF_ENTERTAINMENT">
<pip_modifier value="0.5"/>
</factor>
<factor name="SIF_DEVASTATION">
<pip_modifier value="1.5"/>
</factor>
</factor_modifiers>
<population_levels>
<!-- city -->
<level name="village" base="1000" upgrade="2500" min="500" max="4000"/>
<level name="town" base="2500" upgrade="10000" min="500" max="15000"/>
<level name="large_town" base="10000" upgrade="25000" min="500" max="40000"/>
<level name="city" base="25000" upgrade="40000" min="500" max="60000"/>
<level name="large_city" base="40000" upgrade="62500" min="500" max="100000"/>
<level name="huge_city" base="62500" min="500" max="3000000"/>
<!-- castle -->
<level name="moot_and_bailey" base="1000" upgrade="2000" min="500" max="1500"/>
<level name="wooden_castle" base="2000" upgrade="4000" min="500" max="5000"/>
<level name="castle" base="4000" upgrade="9500" min="500" max="10000"/>
<level name="fortress" base="9500" upgrade="21500" min="500" max="21000"/>
<level name="citadel" base="21500" min="500" max="30000"/>
</population_levels>
</root>