i have bbo in the works and am almost ready for glue up. i have taken my osage stave and worked it down to a 3/4 thick by 2 1/2 inch (roughly) wide plank so to speak. i have straighted it with the heat gun and have the back pretty flat.

my questions...

1 how to get the back perfectly flat for glue up
2 where to go from there

any advice at all would be great please and thank you

karrow

You'll be hard pressed to make a boo backed bow over 1 3/8" wide if the boo is 1/8" at the crown at the handle so that is a good starting point. You can go 1 3/8" at the fades and out 6" to 8" before tapering to 1/2" tips for now or just a straight taper from the fades to the tips. For a 28" draw make your bow 62" to 66" long.
 You can flatten the osage belly with a belt sander.  I would taper the limb thickness a bit so they bend evenly and together before glue up. This will insure an even bend after glue up and I would add about 3" to 4" of Perry Reflex at glue up.

^^ that is very good advice

thank you pat. ill keep workin on her.

I use a toothing plane to level out the back of the Osage and add grooves for a better glue joint. I also do the same to the belly side of the boo before glue up. Your boo should be tapered also from 1/8th thick at the handle to 1/16th at the tips. I also have my boo cut out to the exact shape of the bow before glue up. I also taper the belly of the Osage like Pat says, having the tips 1/8th thinner than the fade area.