Hi Alitsu, the formula f=c/lambda must be used to convert wavelength to frequency, NOT a wavelength spacing to a frequency spacing. In this second case you would need to use derivative to have the relation.
To make it simple: f=c/lambda=299792458/lambda (in vacuum), so for example 1550nm-> 193.4145 THz and 1550.4->193.3646 Thz, the spacing is 0.05THz=50GHz.
To directly convert Df (delta frequency) into Dwl (delta wavelength) you need to use the following: Df = c * Dwl / wl^2, so for example if Dwl=0.4nm and wl=1550nm, you find Df=50Ghz.
I hope I was clear in my explanation, if not just ask!
Alessandro