Just a couple points of reference. Schedule 40 1/2" PVC has an internal diameter of 0.622 (5/8=0.625) so using 3/4 is overkill. The reason 5/8 hose is used is for the Barb fittings. Typically on canister filters with 5/8 fittings the actual hard plumbing, both inside the canister and standard included spray bars is nearly right on 1/2 or 0.500. Your Initial design would have far to much pressure drop, resulting in extremely low velocity from the spray bar (high volume) which would mean very poor flow in-tank. (Scale up if tubing is 3/4 or 1")

What I would recommend is you start by using a pressure drop calculator like this

Pressure Drop Online-Calculator . First measure the actual ID of your filter orifice and included returns. Then calculate the pressure drop based on the unrestricted flow rate of your pump. No need to include head height in your pressure calculation because a canister is on a siphon so head is negligible (1-5"). Then calculate pressure drop across your proposed designs. As long as the spray bar is significantly larger ID than the fixed orifices in your filter then there won't be enough length to add to pressure drop. For instance internal pump may be 0.542 and internal 1/2pvc is 0.622 that is a very significant difference and won't effect pressure drop. So what you want to calculate is the drop across all the individual spray bar holes against their length (0.109" for 1/2pvc). Be careful with hose Barbs!! I have many varieties from many manufacturers and the internal diameter on hose Barbs is no consistent, and always much smaller than hose ID. Hose Barb orifice diameter is a large contribution to this equation. An alternative is to use 3/4" Barb's and heat/stretch 5/8 tube over them.

I would start by aiming for a much higher pressure drop initially, because you can always add more holes. I don't mean this to be an exact science, but it can go you a very good Idea on how many/how big of holes to use. I have made alot of PVC returns, once you get a ballpark idea and a feel for it you won't need all the math. That calculation makes it very fast though. If you need help with putting in or interpreting the numbers let me know.

Wow, did not notice this was a necro post ...