I drilled 34 holes in the spray bar with a 5/32 inch drill bit the stock spray bar had 28 holes at same diameter.
The flow sprays about 2 maybe 3 inches out the holes which I feel is very weak.
I just bought 3 404B's (as best I can tell the 404B and 304B are pretty much the same but the former a bit larger media area?).
I made two spray bars, with a few more holes than you (they are about 20" and holes every half inch). I used a 1/8th inch drill so smaller holes. When I discharge on the top of the water I get a good disturbance for almost a foot (i.e. no scum) and flow for 2' or more. Not great, but adequate.
I measured the flow at about 278 with or without (light) media, far short of the 525, but my Fluval is also far short of it's rating.
My suggestion is just start putting electrical tape over holes until you get what you want, then redo the bar with that number of holes. Or do the math afterwards and reduce the size of the hole to give the same aggregate hole area.
I also made another one for my Fluval 306, and wanted more spray, and found that if you use Schedule 80 pipe instead of Schedule 40 or SDR (I used the latter in my 404B bars) you get a bit more of a nozzle effect as the wall is thicker.
Isn't PVC wonderful - cheap to experiment with.