Transposition for other instruments

Okay, just to make sure I have this correctly, when a key is in C major and I'm playing a song on guitar in C major , A trumpet would play in Bb and a saxophone would play in Eb (or at least a Eb saxophone would). So then for any sort of transposition from one instrument to another you would take the one key and compare it to the other and use that interval to change it. So if you want to take guitar music and change it into something for trumpet you would transpose it down a major 2nd or up a minor 7th.
Other way around, I think. You'd transpose up a major second, because the trumpet sounds a major second lower than written.