I'm trying to cut a triangle frame to the dimensions in the picture. My understanding was that if I wanted the joint to be a 70 degree angle, I would set my miter saw (chop saw) to one half of the desired angle (35 degrees) and cut each piece. I've been trying it with scrap wood, and it doesn't look at all like it should. You can see it in the picture. I'm sure that I'm missing something obvious. Could anyone help me figure this out?

so basically i bought a chess board but it just the board is crooked, i wanna flatten it how i do it🙏