Hi all,
I work out intervals in a similar way to Natalie too. But I may be able to make it a bit more simple.
Always take the note as the Tonic in this case G (forget the # for the moment )
G - B is 3 intervals so it is going to be an x 3rd
G major - the G and the B is natural in the scale (G -

so indicating a major3rd.
This is where u include the sharp which makes the interval a
minor 3rd.
Basically my teacher says always treat it as a major first, look at any sharps or flats and see whether it is major, minor, perfect etc
Hope that makes sense
Fiona