how to find optimum value of a variable d with respect to which Prd and embedding capacity varies

i m working on signal steganography and calculated prd and embedding capacity of a signal at different values of d where d varies between 1:10.the plots of both prd vs d and ec vs d are rising paralleley in the same direction. i need to find the the optimum value of d,at which prd shoud be low at high embedding capacity. i was trying to make an objective function f by attaching weights with prd and ec or plotting ratio of ec/prd but couldnt work. please help