With the Dick O'Kane firing technique you could fire at any angle you wish. The reason for the rule of thumb, picking 10 degrees before he's at right angles for slower speeds and 15 or 20 for higher speeds is that you want the torpedo to turn a very small angle or no angle at all when it leaves your torpedo tube. This makes the most accurate and error tolerant shot possible.
What actually sets the torpedo's path is the TDC, which has your shoot bearing, target speed and angle on the bow. Any shoot bearing you pick will work, it really doesn't matter except that gyro angles close to zero are more error tolerant.
You might want to check out the 3x5 printable cards that explain the Dick O'Kane, John P Cromwell and vector analysis attacks. They are a small, comprehensive checklist that will ensure you don't forget anything.
As far as the range entered goes, it really doesn't matter as a zero gyro shot, or really anything between +-20º, really is considered straight shooting, range drops out of the solution equation. For our game TDC, it is important that a non-zero number is in there or our digital simulation has a cow, but 500, 1500, 2000, any range yields the same shot. This really drives the precise shooters crazy!