Forget about the outside edges of the board, and whether they're bowed a wee bit.
Work off of a centerline.
Lay a weighted single strand of bowstring, or dental floss or whatever, from one end of the board to the other. That's your full length centerline. Under the string, mark your tips and center of the handle. Draw a line to connect the dots. Lay out your front profile on that centerline.
I use big teardrop lead fishing weights tied to either end of a single strand of bowstring over 6' long.
You could also use a long piece of BoTuff fiberglass, a 6' level or whatever. You get the point. Start with a good centerline. Layout your bow from there.