The compound-tunable embedding potential (CTEP) method developed for simulating the influence of environment on a fragment in the ionic-covalent crystal is presented in the form of a linear combination of particular short-range semilocal pseudopotentials for the atoms of nearest environment and the long-range Coulomb potentials from optimized fractional point charges centered on both nearest and more distant atoms of the environment. A pilot application of the CTEP method to calcium niobate crystal, CaNb2O6, is performed. A very good agreement of the electronic density and interatomic distances within the relaxed fragment with those of the original periodic crystal calculation is attained. Calcium niobate crystal can be considered as an idealized fersmite (CaNb2-xTaxO6,x≈0.3) mineral when neglecting the contributions of the impurities with smaller molar fractions, and substitution Nb→Ta is considered here as a point Ta defect in CaNb2O6. Besides, uranium-containing point defects are also studied since the euxenite group minerals, to which fersmite belongs, are considered as prospective matrices for long-term immobilization of high-level waste. The chemical shifts of Kα1,2 and Kβ1,2 lines of X-ray emission (fluorescence) spectra in niobium are evaluated to analyze its chemical state in the crystal. Potential of CTEP for studying properties of point defects containing f and heavy d elements with relativistic effects, extended basis set, and broken crystal symmetry taken into account is discussed.