Beg all you want but the NEC (2017ed) makes no mention of conduit/raceway size when calculating derating. Table 310.15(B)(3)(a) only makes mention of the number of current carrying conductors. You also are supposed to take into consideration ambient temperature. In a residential basement it's unlikely to have an ambient temp requiring additional derating.
Erik,
In all honesty, who gives a shit. Quote current spec all you want. The house was here before you and I were born, the wiring was done and inspected long before you were born, and I'm sure it will be here long after we're both gone. Give a rest son. There has to another book on your shelf.
