Firstly, Ar values for isotopes aren't neat integers but can only be approximated as such. Whereas mass spectra in my textbook seem to have peaks precisely at integer values of m/z. Is this an approximation taken in the drawing of the mass spectra in my book, or is the standard procedure for mass spectrometry to round the m/z values to the nearest integer?

How do we add relative abundances of isotopically different species which contribute to the same m/z peak (i.e. have the same M_{r}) on a mass spectrum?

For instance, if we know that from the parent molecule, we can get "isotopomers" A and B, with the same M_{r}, but where A has an abundance of x:1 relative to the smallest isotopic peak and B has an abundance of y:1 relative to the smallest isotopic peak, how do we find the relative abundance:1 of the peak at this M_{r} value on the mass spectrum?