PlanetsCentral classifies games according to their difficulty using various criteria:

Each of these ratings yields a scale factor, conveniently expressed as a percentage. Multiplied, these give the game's difficulty. A standard Tim-list game has a difficulty of about 1.0 = 100%. A standard PList game has a difficulty of about 1.35 = 135%.

Ship List Difficulty

The ship list difficulty judges the cost of ships in this game. Therefore, it computes the average cost of a ship.

Ship_list_difficulty =
    (Average_ship_cost / 2000)^0.33

The standard ship list has an average cost of approximately 2000, PList has about 5000, PList-heavy has about 10000. Therefore, a ship list has difficulty 1.0 if it is similar to the standard ship list.

Universe Mineral Difficulty

The mineral difficulty judges the mineral richness of the universe, i.e. your ability to obtain minerals for building ships.

Mineral_difficulty =
    (1800 / (Average_surface[T,D,M] + Average_ground[T,D,M]))^0.33

Average_ground[x] =
    (Adjusted_freq[x] * Average_usual[x]) + ((1 - Adjusted_freq[x]) * Average_alternate[x])

Adjusted_freq[x] =
    1 - (1 - PlanetCoreUsualFrequency[x]/100)*0.25

The Average_surface[x], Average_usual[x], and Average_alternate[x] are the average amount of minerals of type x produced by the PlanetSurfaceRanges, PlanetCoreRangesUsual, and PlanetCoreRangesAlternate options.

The PlanetCoreUsualFrequency is adjusted to give more weight to the "usual" values, to compensate the additional logistics required to move minerals from an "alternate" planet to a "usual" one. Also, a planet with 10000 kt minerals isn't worth 10× as much as a planet with 1000 kt, because you'll probably not spend it all.

With this formula, a universe has difficulty 1.0 if its planets provide 1800 kt minerals (=600 kt Tritanium, Duranium, Molybdenum) on average.

Universe Native Difficulty

The native difficulty judges the native population of the universe, i.e. your ability to obtain taxes.

Native_difficulty =
    (25000 / Average_natives)^0.33

Average_natives =
    NativesOnPlanetFrequency * Average_native_clans / 100

Average_native_clans is the average number of natives on a planet that has natives, determined from the NativeClansRange or NativeRange options.

A universe has difficulty 1.0 if its planets have 25000 native clans (=2.5 million natives) on average.

Production Difficulty

The production difficulty judges the production rate host settings, i.e. your ability to make use of the material provided on the planets.

Production_difficulty =
    100.0 / Average_VPI

Average_VPI =
    + 50*(1 + (Average_ProductionRate-100)/100)^0.66
    + 50*Sqrt(Average_MiningRate/100)
    + Sqrt((Average_NativeTaxRate + ColonistTaxRate)/2 * (1 + Average_HissEffectRate/5))*0.8

This is the same formula as used to rate the game difficulty for the VPI, the VGA Planets Player Index.

The Average_Xxx values are the average value of the respective host configuration option. The Average_HissEffectRate is considered as 0 for players who do not have Hisss.

A game with normal settings has a difficulty of about 1.0 here.