There is no "industry standard" answer as far as I know--you can do whatever you want, depending on how much space you have: if it is a tighter space, no angle is acceptable so as to squeeze more table into the formula; if there is a lot of sapce, a longer angle is acceptable. I would avoid going more than 5-10 degrees, though.

What you end up with should depend upon the desired dimensions of your project/table, not a generic industry standard.

Lawrence is correct that there are no standards to adhere to so take the sides before you do anything else and temporarily nail or better yet clamp the back in place to check its comfort. Don't forget that there should be an average comfort range not just suited for some one person......MJH