Linear interpolation is a method of curve fitting using linear polynomials to construct new data points
within the range of a discrete set of known data points.
The combined safe working load (SWL) of the lifting arrangement must take account of the angle of the
slings by applying an angle factor to both the slings and shackles. The angle factors are given
Table 4 in 'Lifting Equipment A User's Pocket Guide' 2011 edition