In my game (house rule, sorry), I use 5 foot increments and even a 2.5 foot increment for medium armor.
So, medium size movement is:
light: 30
medium: 25
heavy: 20
Small size movement is:
light: 20
medium: 17.5
heavy: 15
Then, the character just rounds up for the weird stuff. For example, a small medium armor move is 4 five foot hexes, not 3.5 (a double move becomes 7 five foot hexes). I do not sweat the small stuff there.
The reason I put this in was due to the lack of utility of medium armors. Every character in my game (and every game I played in) used either light armor with high DEX or heavy armor with low DEX. Now, a few characters are actually using medium armor with medium DEX since movement for it is no longer penalized as heavily as heavy armor.