This paper develops a complete physical theory of the Maxwell–Scretching coefficient KKK, the dispersion-dependent quantity that links the universal electromagnetic coupling constant α\alphaα to the measurable composition-resolved refractive slope of double-stranded DNA. Building on the prior QED/MQED/Kramers–Kronig virtual experiment, the paper shows that α\alphaα cancels exactly from the De Ley absorbance ratio R=E260/E280R = E_{260}/E_{280}R=E260/E280, but remains present in absolute optical observables including absorption strength, dielectric response, refractive-index increment, and the DNA composition slope dn/d(%GC)dn/d(\%GC)dn/d(%GC). The surviving relation is written as dnd(%GC)=αK,\frac{dn}{d(\%GC)}=\alpha K,d(%GC)dn=αK, where KKK is not treated as an empirical fitting artifact, but as a physically derived coefficient arising from the one-photon QED interaction, molecular-QED absorption cross-section, Lorentz-oscillator susceptibility, Kramers–Kronig dispersion, and Maxwell dielectric response. The central advance of the paper is that KKK is identified as a wavelength-dependent dispersion functional, K=K(ω),K = K(\omega),K=K(ω), rather than a universal constant. The representative model value at the sodium D-line wavelength is K589≈8.06×10−6 per %GC,K_{589} \approx 8.06 \times 10^{-6}\ \text{per } \%GC,K589≈8.06×10−6 per %GC, but the theory predicts that K(260 nm)≠K(589 nm),K(260\ \text{nm}) \neq K(589\ \text{nm}),K(260 nm)=K(589 nm), making the wavelength dependence of KKK a direct experimental test of the Maxwell–Scretching optical branch. The paper also separates two quantities that can otherwise be conflated: the bulk Ifft/JDCS refractometric calibration slope 10.860110.860110.8601, which belongs to the macroscopic density–refractive-index chain, and the DNA-specific electronic dispersion slope governed by oscillator strength and molecular polarizability. Their separation is quantified by the dimensionless ratio ΛJDCS≈1535,\Lambda_{\mathrm{JDCS}} \approx 1535,ΛJDCS≈1535, showing that the historical refractometric calibration is not itself an electronic-structure constant, but a macroscopic bridge coefficient within the larger Scretching/JDCS slope chain. Finally, the paper derives an uncertainty-propagation budget for recovering α\alphaα from the relation αest=dn/d(%GC)K.\alpha_{\mathrm{est}}= \frac{dn/d(\%GC)}{K}.αest=Kdn/d(%GC). It shows that achieving agreement at the ≈0.0266%\approx 0.0266\%≈0.0266% level, comparable to the previously observed proximity of βs2\beta_s^2βs2 to α\alphaα, requires KKK to be known at roughly the 0.03%0.03\%0.03% level. Because KKK is dominated by the GC–AT oscillator-strength difference, and present molecular optical data are uncertain at the several-percent level, the paper concludes that existing data support a plausible theoretical closure but do not yet constitute precision proof. The decisive test is therefore framed as a pre-registered absolute optical closure experiment in which all constants, oscillator strengths, wavelengths, concentrations, refractive increments, and uncertainty terms are frozen before extracting αest.\alpha_{\mathrm{est}}.αest. The paper closes by presenting this falsifiability protocol as the required experimental standard for determining whether the Maxwell–Scretching coefficient K(ω)K(\omega)K(ω) provides a genuine physical bridge between QED/MQED optical response and the composition-resolved refractive behavior of DNA.