Final set of parameters            Asymptotic Standard Error=======================            ==========================

a               = 0.300001         +/- 0.007744     (2.581%)
b               = 1.54275          +/- 2.4e-05      (0.001556%)
c               = 0.633986         +/- 0.002174     (0.343%)
d               = 4.79335e-07      +/- 1.921e-08    (4.009%)

correlation matrix of the fit parameters:

               a      b      c      d      
a               1.000 
b               0.780  1.000 
c               0.547  0.515  1.000 
d              -0.283 -0.059  0.346  1.000 

sigma = √(d/2)
FWHM = 2.3548 sigma = 0.0011528122958613862
FWHM range [0.0011294758372315894:0.001175685634890722]

visual check: FWHM about 0.001