Re 10% of airspeed+7:
It may be accurate enough, but it's still not very accurate, particularly at low speed. The correct expression should, I think, be arctan(v/364), where v is the speed in knots.
At 50 knots, the approximation gives 12 degrees where the right answer is 8 degrees. At 80 knots, 15, where the right answer is 12. By 130 knots the error is less than a degree. The two agree exactly at only one speed, which is about 160.5 knots.