With nothing better to say - maybe you can just try run a couple of 1D's of guanidine at different concentrations (for example - at the extremes of your set) with a proper chemical shift (like DSS 2,2-dimethyl-2-silapentane-5-sulfonate (DSS) or TSP 3-trimethylsilylpropionate-d6) added and just see what the shift will be?

With nothing better to say - maybe you can just try run a couple of 1D's of guanidine at different concentrations (for example - at the extremes of your set) with a proper chemical shift (like DSS 2,2-dimethyl-2-silapentane-5-sulfonate (DSS) or TSP 3-trimethylsilylpropionate-d6) added and just see what the shift will be?

Gu's H will experience difference in the interaction with water in addition to change of field because of changing magnetic susceptibility, susceptibility (the more guanidinium is added), while DSS will only experience the latter, so Guanidinium will be a poor standard of chemical shift even at constant pH and temperature.