Question about direct feeding external skimmer

[marino420td]

I am about to start the plumbing on my 120 tank. The overflow has a 1 1/2" bulkhead so I will be using this size pvc for the drain. I have this H&S skimmer (A150-F2001)



It has a 5/8" intake.

My question is at what point do I decrease my drain from 1 1/2" to 5/8" to feed the skimmer? I plan on plumbing similar to the ones in this thread.

http://www.reefcentral.com/forums/sh...0&pagenumber=1

Thanks for the help.