You can drop from any of these angles. It just takes proper throttle adjustments and using your best educated guess to determine the bombs path through the air.
Visualize the bomb going through the air before you drop. Your angle of attack and speed will determine it's arc or path. The faster you are going the longer and straighter the bomb will travel. On the other end, Gravity takes hold of the bomb much quicker at slower speeds and pulls it down. It has weight and mass just like a ball thrown in your back yard.... Visualize from there. Use an over hand throw for 90 - 30 degrees and underhand for 29- 0
Dropping from 90 degrees there is really no arc but you risk auguring if not done right. At 0 to 10 degrees the arc is greater depending on speed. It's much easier figure what the bombs arc will be between 35 and 45 degrees. Use the cowl to judge when to drop as Fugitive has said.
(http://wblrd.sk.ca/~realmath_dev/forestry/media/protractor.jpg)