I also had a Gaggia MDF for years and my technique was during active grinding continuously flick the dosing handle and fill the PF as you go. You will need to experiment with the amount you get out, I based mine on a number of seconds once the grinding switch was turned on and got good at guessing when to stop.
Flicking the doser handle also breaks up any clumps while grinding. However an issue with flicking the handle is the dose would load up the left side of the PF requiring a re-distribution (WDT) which quickly remedied that.
A good cleaning is certainly a must. Before I got the Grindz product I took mine apart as its pretty easy to do and there was always old spent beans within the grinder that wouldn't come out without some mechanical means or a good old fashioned vacuuming. Also taking the unit apart allows for a re-calibration procedure which is also very easy to do. You can find instructions on how to maintain it here
http://coffeegeek.com/forums/espresso/grinders/305825I for one found my beans ground best at "2" with a drier bean.
No matter what I always ended up with grinds on the counter when done so I put a dish under the output whilst grinding to catch the particulate that invariably flew out of the grinder. I would use the amount on the dish in my last dose as to not waste it.
A good unit if you keep it clean and don't leave anything in the hopper.