Sling Capacity in accordance with Lifting Equipment Engineers Association (LEEA)

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 below.
Table 4 in 'Lifting Equipment A User's Pocket Guide' 2011 edition

Angle Factors
Maximum load to be lifted = angle factor x SWL marked on sling
Assumes all the slings are the same SWL.
Combination of Slings Angle β of sling legs to the vertical
0° < β ≤ 45° 45° < β ≤ 60°
1 x sling leg or endless sling 0.7 0.5
2 x sling leg or endless sling 1.4 1
3 x sling leg or endless sling 2.1 1.5
4 x sling leg or endless sling 2.1 1.5
2 x two leg slings 1.5 1.0