Option Explicit
Function InterpolateL(lookup_value, known_xs, known_ys)

  Dim pointer As Integer
  Dim X0 As Double
  Dim Y0 As Double
  Dim X1 As Double
  Dim Y1 As Double

  'Following line added to prevent extrapolation
  If lookup_value < Application.Min(known_xs) Or lookup_value > Application.Max(known_xs) Then
    InterpolateL = CVErr(xlErrRef): Exit Function
  End If
  
  pointer = Application.Match(lookup_value, known_xs, 1)
  X0 = known_xs(pointer)
  Y0 = known_ys(pointer)
  X1 = known_xs(pointer + 1)
  Y1 = known_ys(pointer + 1)
  InterpolateL = Y0 + (lookup_value - X0) * (Y1 - Y0) / (X1 - X0)
End Function