We report on the quantitative parameters of fluctuation effects observed in the birefringence and linear thermal expansion of Sn 2 P 2 (Se x S 1-x ) 6 crystals with x = 0 and 0.28. These parameters, including the Ginzburg number that governs the temperature width of the scaling region, are found to be of the same order of magnitude. The relevant implications concerned with the influence of structural defects and multicritical points on the phase diagram are discussed.