For spray starch, the secret is in giving the starch time to sink into the fabric. This probably takes longer if the fabric has not been prewashed because the fabric is treated in the factory to resist dirt (and starch!). Flakes and "snow" occur when the starch is ironed before it has had a chance to be completely absorbed by the fabric fibers.
I do not use spray starch anymore because I seem congenitally unable to wait long enough at the ironing board. Since I don't prewash, my fabrics usually have enough stiffening in them already for accurate cutting and piecing.
When I do starch fabric, it's usually for backing fabric. For that, I mix a 1:1 solution of Sta-Flo liquid laundry starch and water to make a heavy solution, paint it on to the fabric with a large wall painting brush until the fabric is saturated, toss in the dryer, and then iron with steam. Drying the fabric before ironing ensures there will be no wet starch to scorch. With this method I can use a very hot iron and not worry about scorching the starch because it has been entirely absorbed by the fabric. The steam re-activates the starch just enough for ironing all the wrinkles out.