Another overflow question ??

[Just2Bad]

1,38" should be 1 1/4 sch40. I told you 1,38" cause it's what the guy on the phone form the plastics shop told me. But now that I passed by the plastic shop I know that it's 1 1/4". I've never done plumbing before so I'm a really newbe at this.

If my return pump is a mag12 will I loose to much flow if I devide my return in 2 or maybe 3??