The goal of this article is to provide a practical method to calculate, in a scalar theory, accurate numerical values of the renormalized quantities which could be used to test any kind of approximate calculation. We use finite truncations of the Fourier transform of the recursion formula for Dyson's hierarchical model in the symmetric phase to perform high-precision calculations of the unsubtracted Green's functions at zero momentum in dimension 3, 4, and 5. We use the well-known correspondence between statistical mechanics and field theory in which the large cutoff limit is obtained by letting  reach a critical value  c ͑with up to 16 significant digits in our actual calculations͒. We show that the round-off errors on the magnetic susceptibility grow like ( c Ϫ)Ϫ1 near criticality. We show that the systematic errors ͑finite truncations and volume͒ can be controlled with an exponential precision and reduced to a level lower than the numerical errors. We justify the use of the truncation for calculations of the high-temperature expansion. We calculate the dimensionless renormalized coupling constant corresponding to the 4-point function and show that when → c , this quantity tends to a fixed value which can be determined accurately when Dϭ3 ͑hyperscaling holds͒, and goes to zero like ͓Ln( c Ϫ)͔ Ϫ1 when Dϭ4. ͓S0556-2821͑98͒05510-6͔