To improve the performance of TiO2/SiO2 broadband antireflection (BBAR) coating prepared using ion assisted deposition, a hybrid approach was proposed to probe, deduce, and compensate for the thickness errors resulting from quartz crystal monitoring. Using transmission electron microscopy (TEM), a thickness-dependent relative error was found in the prepared BBAR coating. Thinner layer had larger relative error and this was the main reason for the degradation of the performance of the BBAR coating. By fitting the thickness errors via a linear model, the origin of the thickness-dependent relative error is determined to be a constant thickness offset of the quartz crystal monitor. The thickness offsets are 1.5 nm and 2.7 nm for TiO2 and SiO2, respectively. Finally, an approach through reverse engineering of a 4-layer coating was proposed to obtain the offsets for both materials. The results achieved are comparable to that obtained using TEM, but this approach is much more convenient. After compensating these offsets, a BBAR coating was prepared with transmittance higher than 99% from 450 to 900 nm.