CSS3 - transform:translateY( out of screen )

I am trying to add a slide up from down.

And adding starting position:

    transform:translateY(200%) 

which works fine if the scroll area is not too big.

    transform:translate(-200%) 

works better because it moves relative to the top and that part can't grow.

Is there a good way around this?

Using

   transform: translateY(50000px);

and will potentially affect animations as they have to move over larger areas.

What is a good approach to this problem?