This is what I used to make really straight buttonholes. I read this at a blog or at patternreview somewhere. I can't remember where I saw it. It is not my idea. If you know where I saw it, let me know and I'll link to it.
I use tape and stick it to the approximate location on my jacket. I use my quilting ruler to make sure that it is perpendicular and straight compared to the edge of the jacket. Then I draw on the tape with a pen. This way I can easily see the ends of the buttonhole when I am sewing.
Here I have sewn two out of the three you can see.
Here is the finished product with the brass buttons. I love it.