For some nuclei with I=1/2 like ¹H, ¹³C or ¹⁵N you can make use of a collection of chemical shift tensor values for certain molecules. According to your selection Weblab will set the associated principal values⁽¹⁾ as known from literature^(2,3). Note that not only the principal values but also their orientation with respect to the cone axes system is of crucial relevance for dynamical studies. Here you have to find the correct choice by yourself. You must consider that the cone angle θ used by Weblab is always the angle between the cone axis and σ_zz and, since most shift tensors are not axially symmetric, you will have to supply a value for the euler angle α. The following pictures, also taken from the reference above, may give some helpfull hints.

(1) 1H: values are relative to a spherical sample of TMS, 13C: values are relative to TMS, 15N: values are relative to NO_3>-

(2) Spiess H. W, (1978). Dynamic NMR Spectroscopy. NMR Basic Principles and Progress, Vol.15 p.158 and references therein.

(3) The tables in ref.(2) contain the principal values of the shielding tensors.They are converted to the corresponding shift tensor values. In addition the ¹³C data are given relative to CS₂ and are converted to TMS by using a value of 193 ppm for the isotropic chemical shift of CS₂ relative to TMS.

(4) Values for phenylene are taken from Hansen R. M., Graf R. and Spiess H. W. (2012), Solid-State NMR in Macropmo9elecular Systems: Insight on How Molecular Entities Move, Accounts of Chem. Res. 46,1996