Opacity Question

OK, I've learned a few tricks about making an image look as though the subject is ballooning. My eye, however, is terrible when it comes to some bits liek opacity. I know if I go below 40% for the intervening layter it looks like a ghost, and if I go above 70% the behind image shading looks too crude. Which opacity then, by opinion, would be best suited for this level of inflation? ( I know that as it gets bigger you use more opaque, I'm just looking for an opinion guideline for the final work... which will contain a process of intervening steps. )