  We investigate the cross-over temperature T* as a function of doping in
(Ca_{x}La_{1-x})(Ba_{1.75-x}La_{0.25+x})Cu_3O_{y}, where the maximum Tc
(Tc^max) varies continuously by 30% between families (x) with minimal
structural changes. T* is determined by DC-susceptibility measurements. We find
that T* scales with the maximum Neel temperature TN^max of each family. This
result strongly supports a magnetic origin of T*, and indicates that three
dimensional interactions play a role in its magnitude.
