Try looking at "Standard Potentials in Aqueous Solutions" by Bard, Parsons and Jordan. There is not direct answer but you have there a set of relevant equations that you should be able to calculate it

Thanks for your answer. I have also checked for the solution in Bard's book but could find the same. I appreciate your effort.

You can also meaure by yourself. For example, using Pt (or GC) as working electrode and 1 M H2SO4 as electrolyte. The problem in your case could be that the reduction of Pt(NH3)2+ to Pt(0) could become an irrenversible electrodeposition process with overpotential needed which depends on electrode materials. Are you sure the electrochemical reaction in your case is reversible?