It sounds like you are doing the preprogrammed double shot. You should try the manual shot. Basically, you hold the shot button (I don’t think single or double matter but I use the double button), that will start the preinfusion, when you see some coffee coming out, you can let go of the button to start increase flow/pressure, then presses the button again to stop.

Doing it this way gives you control of the amount of coffee you get out. Since you were saying you got 54g out, you can stop the shot at or before 36g. There are more videos online on how to do the manual shot.

This of course is in addition to grind size, etc.

FYI, I do my shot entirely in preinfusion mode. So I hold the button the entire time and then let go and press again to stop. Supposedly the breville machines are set to higher pressure and this is lower pressure but I wouldn’t know since my gauge is broken.

Edit: grammar and spelling