It's complicated. There are four base cum-related stats:
- Cum Multiplier, which is split up into Raw and Mod (former is permanent, latter is temporary). It acts as a multiplier to Probable Ejaculation, and is called "Cum, Quantity Modifier" in the Codex (the internal name is cumMultiplier(), comprised of cumMultiplierRaw + cumMultiplierMod, which is why the Wiki and I call it Cum Multiplier; IDK why they call it a different name in the Codex).
- Ball Efficiency, which is supposed to represent the number of "standard/regular" cumshots per day a character can hold (default 3) and is intended to be ~3x Cum Multiplier. It acts as a multiplier to the maximum amount of cum you can store in your balls/prostate, and as a divisor to Probable Ejaculation. It's not listed anywhere in the Codex.
- Refractory Rate, which determines how fast you produce cum over time (each point gives 2% Ball Fullness per hour, doubled with the Breed Hungry perk and halved for Ball Fullness >= 60%, plus other modifiers if a Cum Cascade is active). Certain thresholds will also increase the minimum amount you can cum. It's listed in the Codex under the same name.
- Ball Fullness, which is a percent equal to how much cum you currently have with respect to your Max Cum.
The base Probable Ejaculation is equal to Cum Multiplier x Max Cum x Lust Coefficient* / Ball Efficiency, which is then increased by 50% if you're in Rut, then capped at your Current Cum (i.e. Ball Fullness x Max Cum), rounded to the nearest 10mLs, increased to minimums based off Refractory Rate, certain perks and Rut-type status effects, boosted by the Opal Cock Ring if equipped, capped at your Max Cum, increased to Current Cum if you have overloaded 'Nuki Nuts, gains a bonus from stored biomass if you're a goo and have that option enabled, and finally increased to the value of the MinCumQ perk if it exists. Upon orgasm, this value is subtracted from your Current Cum to determine your new Ball Fullness.
*a percentage equal to 75 plus half your current Lust