  We evaluate multi-parameter functions by fixing all parameter values, but
two. The temperature power spectrum of the lensed cosmic microwave background
(CMB) serves as an example in which the amplitude, $A_{s}$, nears linearity at
small deviations, reducing power spectra computation to a 1D problem. We apply
the analysis to assess the shift in the apparent value of ${H_0}$ as a result
of cosmic infrared background (CIB), and Poisson point sources (PS). By
iteratively cycling through parameters paired with $A_{s}$, and within a few
hundred calls for spectra, we derive values in agreement with Planck. We
adjudge that when neither variable is linear, thousands of calls are required,
still competitive with the MCMC method.
