Easiest Way To Do This?

[crokett]

This is a side view of the soffett for the bench project. The arc will be formed by 1/2" ply cut with kerfs. The sides will be rabbetted to receive the arc and the flat end pieces. I will rough cut the arc piece long so it can be flush trimmed once I know how long to make it. Question is, what is quickest way to get the ends of the arc flush to the top of the side pieces (in red)?

[Duff]

I'm probably reading this wrong, but I'll offer just in case. . .
If you know the top corners are perfect 90', how about running it through the saw with one of those top corners against a HIGH fence attachement? That would (if you're blade is set to 90' also, make the red lines parallel w/the top (currently on the side). . .

[crokett]

Ya know that is a good idea. I want the arc piece to sit in a rabbett but there's no reason the end pieces have to.

Thanks!