Hi Makman,
This post is back when I was using these methods as shooting methods, that is, to derive a gyro angle. I now use these methods to derive target speed (see my stuff in my sig as well as this that I made for Wolfpack
https://drive.google.com/file/d/1isf...w?usp=drivesdk. That is a more evolved way of using the methods and is more in line with historical procedures.
To your question, the setting of 90 is simply for simplicity’s sake. Setting 90 allows you to set the target speed = own speed in the TDC, that’s all. Since the formula is
own speed x sine(brg) / sine(AOB) = target speed,
Then setting TDC speed to equal own speed yields the same result since the sine of 90 is 1.
Again, antiquated method and superseded by my subsequent stuff.