There's no maximum gap, though a smaller gap will produce a thicker mouth feel. There is a minimum gap, however. You never want your coffee puck to hit the screen when dry. The easiest way to get a good gap is to prepare as normal, grind dose tamp etc, but we're not going to pull the shot. Carefully place a 5c coin on the coffee puck where your shower screen is at its lowest (with most machines this is in the middle, but it sounds like yours is at the edge). Lock the portafilter into the machine but don't start a shot. Remove the portafilter, and as carefully as you can remove the coin without disturbing the coffee. If there is no mark from the coin or a very slight impression your coffee puck height is ok/not too high. If the coin has been pushed into the coffee your coffee puck is too high. You risk damaging your machine and are causing excessive wear every single time you pull a shot like this. If there is a screw or bolt head sticking down into the puck you might be able to change it out for a countersunk head or even a flatter head.
If the coin clears the shower screen but you're seeing an impression of the shower screen and/or screw in the puck after pulling a shot, don't worry. The puck stays compressed until the pressure is relieved at the end of the shot. Check out the glass portafilter videos on YouTube to see it happen live.
If your shower screen is high in the middle and low at the sides, I'd keep your puck flat and just lower than the sides. What machine have you got?
If the coin clears the shower screen but you're seeing an impression of the shower screen and/or screw in the puck after pulling a shot, don't worry. The puck stays compressed until the pressure is relieved at the end of the shot. Check out the glass portafilter videos on YouTube to see it happen live.
If your shower screen is high in the middle and low at the sides, I'd keep your puck flat and just lower than the sides. What machine have you got?

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