For FB I made slots 0.08" wide, which makes 15% open area.
Minimum, but I sincerely hope that"ll work because if I intend to fill it with holes it will be pretty PITA without table drill.
It is still under construction and I"ll wait until I weld connections on keg before soldering it.
I used part of keg valve with some copper reducers to seal center for dip tube, it should be fine.
It took me couple of hours to cut slots with angle grinder. I used clamps to hold bottom in place and piece of wood batten to keep straight lines.
When cutting slots on SS it is good to cut layer by layer until we cut it out, this way the slots will be cleaner.