Let's examine this a tad.....
366.6 Number of Conductors.
(A) Sheet Metal Auxiliary Gutters. The sum of the cross-sectional areas of all contained conductors at any cross section of a sheet metal auxiliary gutter shall not exceed 20 percent of the interior cross-sectional area of the sheet metal auxiliary gutter. The derating factors in 310.15(B)(2)(a) shall be applied only where the number of current-carrying conductors, including neutral conductors classified as current-carrying under the provisions of 310.15(B)(4), exceeds 30. Conductors for signaling circuits or controller conductors between a motor and its starter and used only for starting duty shall not be considered as current-carrying conductors.
(B) Nonmetallic Auxiliary Gutters. The sum of cross-sectional areas of all contained conductors at any cross section of the nonmetallic auxiliary gutter shall not exceed 20 percent of the interior cross-sectional area of the nonmetallic auxiliary gutter.
--note the 30 conductor rule, at 29 conductors there would be no derating, on the introduction ot the 30th, the whole shebang is only good for 45% .
the 20% ( or as in this pix -366.9(A)'s 75%) max fill
However, this dooes NOT apply to non-mettalic gutters. The same language can be found in wireways ( 376, 378)
--the 20%-75% fill max. Where 250 or whatever amount of conductors were to occupy a 5% fill, or be ventilated,would not the condition of captive heat change? There are plenty of power vented motor control cabinets out there, stuffed to the gills, and an 8x8 could easily be speced as a 12x12 or larger gutter.
---these conductors typically come from a panel which is not held to any form of derating ( at least for # of conductors) , then through nipples also exempt-less than 24". If 310.15(b)(2)(a) were to be enforced and say #8 conductors run from breaker to gutter/wireway, the existing circuits would also need this applied, all pipework, rx, etc etc would need be changed.
......all in all, the derating factors seem fixed and unalteralble , there is no format to quantify as to adjustment. Maybe this is why some use old panels as JB's?