Lisa,
I asked my wife about the button holes yesterday. The shirt she had on had horizontal holes. Others have vertical. I honestly never had paid attention. I've never made a shirt or jacket or anything with a button hole. But the button holers all seem to make them horizontal. At least their instructions seem to tell you to orient the material that way.
I've never used that little ruler. But it looks like a useful tool. Since I started sewing about 6 months ago I've learned I have too many things to work with (play with) and too little time to do it.
Joe