The functional form is identical to that in CAL_getRGAIntercept
(section 3.4.29), except that the tabulation
of is used for the efficiency. is scaled for
off-axis sources identically to function CAL_getRGAIntercept,
is tabulated in CCF QuantumEf for on-axis sources, and is interpolated for different off-axis values by this equation. The remainder is identical to function CAL_getRGAIntercept.