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This page describes how rounding appears to work in Fallen London. Note that this is based on observations, so if something appears inaccurate feel free to contribute.
Quality levels[edit source]
While most quality unlock requirements and change amounts are a set number, sometimes a more complex formula can be used. If this involves division or multiplication by a fraction then the resulting number is rounded.
In most cases, rounding behaves how you would expect. A number with the decimal part smaller than 0.5 is rounded down and a number with the decimal part greater than 0.5 is rounded up. If the number's decimal part is exactly 0.5 then the number is rounded to the nearest even number.
For example, when hunting a large rat (after acquiring the first level of A Name Scrawled in Blood), the action Expose yourself as bait determines the challenge difficulty level by multiplying the Quarry's Savagery (15) by 1.5 to get 22.5. Usually this would be rounded up. However because 22 is even, in this case it is rounded down.
While this may appear counter-intuitive, this is a standard implementation of rounding in much of modern software and presumably is also implemented in the software used by FBG.
Challenge success chance[edit source]
The challenge success chance seems to be treated differently. In all cases it is rounded down, even if the decimal part is greater than 0.5.
For example, in the example above, a player with Dangerous 15 will have 0.6*15/22 = 40.9% chance of success. Usually this would be rounded up but in fact it will be rounded down to 40%.