= Solution
Place a far detector near the first atmospheric <Neutrino oscillation>[disappearance] maximum, approximately $1.2\,\mathrm{km}$ for the source's monochromatic <energy> and splitting, and a nearly identical near detector a few hundred metres from the <Nuclear reactor>[reactor] complex. Their flux-corrected comparison largely cancels <Nuclear reactor>[reactor] power, emitted-spectrum and absolute-efficiency uncertainties. For a broad <Nuclear reactor>[reactor] spectrum one fits energy-dependent <Neutrino oscillation>[disappearance] instead of treating that single baseline as a universal exact optimum.
Detect $\bar\nu_ep\to e^+n$ in a <Inverse beta decay>[hydrogen-rich target], for example <liquid scintillator> with neutron-capture enhancement. The prompt <Positron> signal estimates <neutrino> <energy>; the delayed neutron-capture signal supplies a time-and-position coincidence that strongly reduces random backgrounds. Use sufficient target <mass> and exposure, low radioactivity, overburden and a <Active detector veto>[muon veto] to reduce cosmogenic backgrounds, <Calorimeter calibration>[calibrated] relative <energy> response and efficiency, and monitored <Nuclear reactor>[reactor] powers and baselines. Account for multiple <Nuclear reactor>[reactor] cores, spent-fuel backgrounds and accidental coincidences. At kilometre baselines the slow solar <phase> is small, so the <Neutrino oscillation>[disappearance] depth directly probes $\sin^2(2\theta_{13})$ with little sensitivity to the <CP violation>[CP phase] or <Neutrino mass ordering>[mass ordering].
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