As a third-year owner of a Gaggia MDF Coffee Grinder, I'd like to share a few of my not-so-secret tips for using this machine.
Since this machine has 34 settings, I'll save you quite a bit of time (and coffee) and say that the best grind setting for espresso is going to be right around a number 6. Of course, there are going to be variables to this setting, based on the type of coffee or espresso machine being used; but a 6 would be a good starting point. You'll be able to adjust up or down a number or two from there. The first time I used the MDF I started at the halfway point, around an 18, and then adjusted down one setting at a time from there. After a well-spent hour, I can now share my experience with you in hopes that you'll have a quicker and easier time dialing in your MDF.
I also know from personal experience that this grinder can actually grind too fine, basically turning your coffee puck in to cement in your portafilter. Quick word of wisdom: Don't run your espresso machine for more than 10 or 15 seconds if there is no water coming out of the portafilter, as you run the risk of ruining your pump.
I use my MDF once a week to grind a pound of coffee for my drip coffeemaker. As most people know, drip coffee requires a courser grind than espresso. In this instance, I find that a 15 grind setting on the MDF works best. Most might consider the resulting grind to be too fine for drip coffee; but I feel I get a nice, rich, full-bodied extraction out of my drip coffeemaker with this grind setting.
In a nutshell, the best grind is going to be whatever suites your palate; so practice, taste test, and enjoy! Do you have a Gaggia MDF at home? If you do, share your favorite grind settings with us.