The semi-tone idea is ok to start off with, but in the long run, I believe the better way is to just learn the intervals. If you know the major scales, intervals should be a piece of cake.
For example, when I see C-E, I automatically see a major 3rd. If I had to see 4 semi-tones, I would drive myself crazy. The inverse of a major 3rd is a minor 6th. So, by learning all the major 3rds you should have simultaneously learned the minor 6ths. Cuts the learning time down quite a bit using this method.
In the end, whatever method helps you to learn best is fine.