Custom Previous Button

I would like to use my own next and previous buttons rather than the ones in the player.  I have them in a master slide so they appear the same on all slides and can be revised in one location.  The next button works great.  The problem is when someone is skipping around through the course using the menu the previous button goes to the last slide visited. 

For example if you start at page 1, go straight to page 5 using the menu and then hit previous you land on page 1.  I would like it to go to page 4.  Obviously I can put the previous button on each individual slide and hardwire it to go to a specific slide, but when the course gets to be 80 slides long this becomes pretty unmanageable.

The next button uses the slide numbers and goes one higher.  I want the previous button to do the same thing in reverse (go to one slide number lower).  Is there a way to do this that I'm missing?