How do you calculate/compute/determine the spectral coverage of a frequency spectrum?
Spectral coverage, I assume, means the Bandwidth required to transmit the incoming signal. A simple formula(approximate) for the Spectral Efficiency of Memoryless Modulation Schemes(MSK is not one) which gives the ratio between Data Rate and Bandwidth required is:

R/W = (2*logM)/N


where R is the incoming Data Rate and W is the required Bandwidth; log is in Base 2, M is the number of points on the Signal Constellation and N is the number of dimensions required to use the particular signalling.


FSK requires as many as Dimensions as the number of Points you want to transmit.
ASK is one-dimensional.
QPSK is two-dimensional.
MSK is more spectrally efficient than the other 3 and I think the formula for it's Power Spectral Density and its 3dB value can be found in all Digital Communication Texts.