Fresnel zone plates (FZPs) are widely used in X-ray microscopy (XRM) for their high spatial resolution and compatibility with full-field imaging, but their strong chromaticity poses challenges under broadband illumination such as that from laser-produced plasma sources. A common monochromaticity criterion, $\frac{N_z}{\zeta} \leq 1$, where $N_z$ is the number of zones and $\zeta$ the source monochromaticity, is often cited as a limit for negligible chromatic aberration, yet its quantitative implications for both resolution and depth of focus (DOF) remain poorly defined. Here, we introduce the zone–bandwidth ratio $Y = \frac{N_z}{\zeta}$ as a compact, design-relevant parameter that isolates the effect of the zone plate from other system complexities. Using Fresnel wave propagation simulations with radial Hankel transforms, we quantify resolution and DOF as functions of Y for Gaussian spectral profiles. As the work suggests, the number of zones does not have to be as high as 900 to generate the results, it takes a bit longer, so anything above 120 zones should produce very ideal results without artifacts of low zone numbers. The same goes for the constant focal length, considering low zeta numbers and that the zones climb quite high via $Y$, $\zeta = 120$ is perfectly fine.