Weenie time...
An equation for determining the appropriate bank angle in a turn involves TAS and rate of turn. For a standard rate:
bank angle = acos( 1/ sqrt( TAS^2/132637 + 1 ) ) * 57.3
The rule of thumb, TAS/10 + 6 (not 5), is a linear approximation that offers a surprisingly good fit out through about 560 knots.
Up next, "Why plan a 3° descent?"