I agree with Mark, trying to get any kind of "gentle" grade when the grade change plywood is butted into the flat section is a nightmare. Not sure about the 5/8th ply, it is going to be stiffer than the 1/2 or the thinner stuff they sell as 1/2. I could get quite a slope out of the thin stuff - but didn't really need that much flexibility. Just cut a section and see what constraints you will be working with. It will take a little more cutting time.
Kind of obvious, but here are two images of split plywood. One where I have not filled in the seam and the other where I am partly done. Having done both methods, I will never, ever, do another butt joint where I am starting to change grades. I have messed up enough plywood cutting in the wrong spot or wishing I had laid out the track differently. Once cut you are pretty much committed, unless you are willing to replace and re-cut. As Mark kind of said in another post - I cut that piece 5 times and its still too short! or too narrow! or the wrong curve shape! Been there done that too many times. ; )
