Kramers-Kroenig (K-K) analysis of harmonic generation optical data is usually greatly limited by the technical inability to measure data over a wide spectral range. Data inversion for real and imaginary part of $χ^{n}(nω; ω, >... ,ω)$ can be more efficiently performed if the knowledge of one of the two parts of the susceptibility in a finite spectral range is supplemented with a single measurement of the other part for a given frequency. Then it is possible to perform data inversion using only measured data and subtractive K-K relations. In this paper multiply subtractive K-K relations are, for the first time, presented for the nonlinear harmonic generation susceptibilities. The applicability of the singly subtractive K-K relations are shown using data for third-order harmonic generation susceptibility of polysilane. Comment: 11 pages, 2 BW figures