I looked at the width of the sub-terminal tail band on adult Red-tails as well as the darkness to the red color and found that the smallest Red-tails (ones that take 7A bands and weighed under 900 gms healthy) tended to have broader sub-terminal bands and darker reddish tails than "large" Red-tails (weighing over 1200 gms and requiring 7D bands). The real issue (besides overlap in these traits) is that most Red-tails fall between these measurements and show a sub-terminal band intermediate in width! Another issue arises when comparing light-morph to dark-morph birds, birds from different geographic locations, or factors that could effect tail color (fading, etc.).
By the way, there is overlap in the sub-terminal tail band width of adult male and female Swainson's Hawks as well, so do not use that trait on its own to sex Swainson's Hawks, especially when sexing dark-morph birds! I should also mention that there is a paper on sexing Western Red-tailed Hawks using a combination of measurements authored by Kara Donohue that you can download here:
Presumed male Eastern
Presumed male Western
Presumed female Eastern