+1 to slightly overfilling. The first time I bled the SRAM level brakes on my MTB I was having trouble pressurizing the system which made my levers feel a bit spongy. I was fairly confident that I got all the large air bubbles out, so I ended up using a hex key that was slightly narrower than the bleed block to overfill a bit and it worked quite well - though, something plastic would probably be safer to ensure you don’t damage your pistons.
Another option that I’ve seen recommended is to pressurize the system from the caliper end rather than the lever end (assuming you have a bleeding edge port). I think I’ll try this next time since overfilling is a bit finicky to get right.