Ok, this is based only on my experience with a Lelit PL041EM, and a sunbeam EM0480, which was then replaced with a Mazzer Super Jolly.
1) Give up on the single while youre learning - while it is a good single basket, its very difficult to obtain consistency.
2) The EM0480 you can easily go from OK shot to choking on this machine in one click. This can mean as the coffee is ageing (and generally you want to grind finer as it loses freshness), you will find yourself searching for a spot between two clicks.
3) On a double, aim for around an 18g puck - seems to be most consistent (for me)
4) Get two measuring cups and split the shot in two. This will a) make sure you are getting pretty close to the right amount and b) make sure you are tamping evenly.
Note point 2 has been solved for me completely since upgrading my grinder.
Temperature is the other thing that can affect the shot considerably, as the temp varies a lot, and I feel it runs a touch cool (but this probably varies from machine to machine). Some people wait until the light turns off, then back on to ensure its at a stable temperature before pulling their shot. I have a different technique, but dont think its something that will help you initially.