working point elevation of closed drain header

Sir,
What will be the starting working point elevation of a Sloped Closed drain header so that we can calculate the elevation of sump drum in to which the header ends.

Sorry bud, but I have to say.....this is a really bad question. You might as well be asking JOP to do your job.

The answer to your question if you know what your actually modelling or drawing (2D) should be obvious. I'm assuming it's below grade piping. If in doubt cut some sections.

Your slope will start at the farthest drain. It will be a set slope on your P&ID's (i.e. 1/8" per foot) all the way to the location of your sump. Your farthest drain will set the elevation because depending on your location in the world you will need to bury the pipe a set distance mininum below grade. For example here in Canada, the frost line is 4'-0" below grade compared to the Texas panhandle which may only be 2'-0". As you slope your lines...the further away your sump is the deeper it will get and the longer your vent lines, pump outs etc will need to be off your sump. You'll need to consider that when you locate your sump.

Hint: Your question might read better as.....
What is the min. below grade depth I need to my top of buried pipe in "give the location". That will get you a min. starting point elevation. Then make you elevations a nice round number from that point.