Thanks for clearing that up Young Col.
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If you were confused previously, ignore the below! (excluding old gits)
Old git, to be pedantic if there are 2 semi-tones between Bb and C on tenor and 3 on alto then there should be 2.4 on a C mel. Explanation below.
alto in Eb has 3 semi-tones between Bb and C
tenor in Bb has 2 semi-tones between Bb and C
5 semi-tones between tenor (Bb) and alto (Eb), therefore 1 semi-tone difference between Bb and C for every 5 semi-tones difference between instrument keys.
Thus, C melody sax in 'C' between alto and tenor. 2 semi-tones higher than Bb tenor. Therefore 2/5 of a semi-tone. so really it's 2.4 semi-tones between Bb and C on a C melody.
Also working the other way, 3 semi-tones lower than the alto implying 3 - 0.6 = 2.4 QED.
Note: That's why my B is always sharp on the C-mel!