Formal justifying act is complex — intellect and will informed together
So the formal proper act by which saving faith justifies should be stated more generally — as including several particular acts that constitute that faith.
Most Reformed doctors agree: justifying faith is not one simple habit or act, but a complex of acts/habits informing intellect and will together.
Latin
CXII. Itaque puto formalem & proprium fidei salvificae actum, & ratione cujus justificare dicenda est, generalius concipi & exprimi debere; & ita ut in se includat & involvat plures actus particulares fidem illam constituentes. Quandoquidem, ut jam est a nobis observatum, ex consensu maximae partis Doctorum Scholae Reformatae, fides illa non est unus simplex vel habitus, vel actus, sed complexum plurium actuum, vel habituum, qui non solum intellectum, sed ipsam quoque voluntatem informant.