If you have accurate scales, weigh the dose and see if it comes out anywhere between 14 and 18 grams for a double. Thats a good start.
The number of fill/tamps youre going through should well and truly choke the machine. Since thats not happening, sounds like the grind could be too coarse.
Where the grounds come to beneath the lip is not as important as how tightly its been packed to that level. If you are having trouple locking the portafilter, that should indicate the dose is too heavy.
Load the basket naturally without pressure until its full. Then give it a tap on the bench to settle the grinds --again, without tamping.
Then scoop some more coffee in until its heaped. Level it off with a straightened finger. Tamp lightly. Some grounds will stick to the sides above the dry puck, so give the basket a gentle tap on the side to knock these off the inner wall.
Now, Tamp with force to produce a straight, even puck.
Lock and pull a shot: 25 - 30 secs for both single or double.
If youve done all the above right and it gushes out way under time, grind finer. If youve done all the above right and it takes way over 30 seconds to trickle out ....grind coarser.
In any case, experiment with the double basket -- it should be easier than the single. Intitially you may waste a little more coffee getting all the variables right, but the learning curve will be steeper.