As far as I know, Powerpoint won't let you animate a scale that precisely. What I do, and I'm not saying this isn't insane, is if I need an animated effect that isn't available, I actually create it in another software, usually After Effects, export as JPEG or PNG sequences, bring them into Powerpoint and animate them image by image using Appear after previous. I know... wow. But that's not all... if I have to have the slide look good for printing? I layer the background shape from the master over the slide, crop it where I can, and then situate the final build on top of that so it looks good. The background shape, you set to Disappear as soon as the slide is played. That being said, if the option you mentioned of using many separate slides works, then that's the best way to go.