A agree in a way that I am also not a fan of the many "great results with stupid C41 modifications"-threads on APUG.
Originally Posted by Gerald C Koch
But: C-41 is usually a constantly running machine process with a seasoned developer, defined replenishment, nitrogen burst agitation, and complex process monitoring. Home processing C-41 will always be just a rough approximation (at best). Under these circumstances, additional dilution of 10% will hardly have any (visible) effect. Especially as C-41 is quite a "strong developer" where slight variations in concentration will have only a minor effect.
Simply add 50ml of a.d. to your 450ml kit (do not mess with too few volume, it's not worth it). Probably add a few seconds (5-10), but not more, to avoid over-compensation.