i have the same combo - classic + mdf. Im currently using a blend (no idea what it is) from my local roaster in the blue mountains. Over the past month ive changed beans several times, changed my pressure and my steam wand so ive been re-learning a lot.
basically, I ignore the weight recommendations - the espresso you produce is far more important than how much coffee you use. Instead i wasted a few hundred grams and timed my grind and compared shots. im currently grinding on 5 for 23.5 seconds and producing consistent, delicious espresso. Ive maintained this consistency by using the same beans for several weeks/kilos, but im sure ill have no problems translating the technique to new beans. tamp is tamp is tamp - do it a hundred times and do it the same way - so long as its suited to the grind size and its producing the right results then dont change.
i have noticed, after lovering the pressure, that the machine is far happier with fresh beans - the stale fairtrade roasts and the emergency bag of lavazza came out terribly, but the fresher roasts are beautiful.
so, my recommendations are things you already know - fresh beans, consistent tamp, but dont worry about weighing beans (im sure youve looked inside your machine and seen how much of you grind doesnt actually make it to the doser), instead use grind time as your measuring stick.
good luck :)