Here is the link to the original paper (I'll update this answer once I read . A 10ms gaussian pulse calibrated for 90 degree on-resonance flip angle will introduce almost no excitation at offests >300 Hz.
So if you apply gaussian pulse like that for selective excitation the article).closest other resonance must be farther than 300 Hz away. If you need better selectivity - then just proportinately increase duration of the pulse and re-calibrate it again so that it has 90 degree flip.