The amount of coffee that is optimum for your particular setup will depend on the basket volume (measured) and other factors.
Best way to determine this is via the use of the so-called and ubiquitous "Five Cent Test".
Basically, prepare a shot in the usual way and after tamping, gently place a five cent piece on top and in the middle of the prepared coffee puck. Lock the Groupe Handle into place as per normal and then remove the Group Handle and observe whether the "zack" has been disturbed. If it has, then you have dosed an excessive amount of coffee into the filter.
Knock the puck out and prepare another one with slightly less coffee and test again and keep repeating until such time as the zack remains undisturbed, thereby indicating that you have clearance between the top of the coffee puck and the bottom of the shower screen.
The converse of this procedure naturally applies if your coffee dose is insufficient...
Anyway, once you establish the correct dose for the filter you have, weigh it and make note that this will be the maximum dose with the particular coffee you are using. Different coffees and different roast styles/depths will vary a little bit so to be safe, you may want to reduce this noted weight by 0.5g or so to account for the variability. Optimum pour rates can then be adjusted by grinding the coffee beans either finer or coarser.
Hope this helps,